Exhibit 10.13.3

                          FIFTH MODIFICATION AGREEMENT


     BY THIS FIFTH MODIFICATION  AGREEMENT (the  "Agreement"),  made and entered
into  as of  the  15th  day  of  December,  2003,  WELLS  FARGO  BANK,  NATIONAL
ASSOCIATION,  as administrative agent (the "Administrative Agent") for the Banks
listed in the  hereinafter  defined  Credit  Agreement  (the "Banks") and as the
Issuing  Bank and the Swing Line Lender,  and KNIGHT  TRANSPORTATION,  INC.,  an
Arizona  corporation  (the  "Company")  and all present  and future  Significant
Subsidiaries of the Company (with the Company, the "Borrower"), in consideration
of  the  mutual   covenants   herein  contained  and  other  good  and  valuable
consideration,  the receipt  and  sufficiency  of which is hereby  acknowledged,
hereby confirm and agree as follows:

SECTION 1.  RECITALS; ACKNOWLEDGEMENTS.

     1.1 The Borrower and the  Administrative  Agent and the Banks  entered into
that Credit  Agreement  dated April 6, 2001 (as amended  from time to time,  the
"Credit  Agreement")  to provide  financial  accommodations  to the  Borrower as
provided  therein.   The  Credit  Agreement  was  previously   amended  by  that
Modification  Agreement  dated  as of June 5,  2001,  that  Second  Modification
Agreement dated as of November 19, 2001, that Third Modification Agreement dated
as of February  13, 2003,  and that Fourth  Modification  Agreement  dated as of
September 15, 2003.

     1.2 Borrower and the  Administrative  Agent, with the consent of the Banks,
desire to modify the Credit Agreement as set forth herein.

     1.3 All  undefined  capitalized  terms used  herein  shall have the meaning
given them in the Credit Agreement.

SECTION 2.  CREDIT AGREEMENT.

     2.1 The following  definitions  in Section 1.1 of the Credit  Agreement are
hereby amended to read as follows:

          "Loan  Documents"  shall mean this Agreement,  the Notes, the Security
     Documents and all other documents, instruments and agreements of every kind
     and description at any time undertaken by any Person for the benefit of the
     Banks in connection with the Loans.

          "Maximum RLC Commitment" shall mean $10,000,000.00.

     2.2 The following  definitions  in Section 1.1 of the Credit  Agreement are
hereby deleted:

          "Refunded Swing Line Loan"

          "Swing Line Balance"

          "Swing Line Commitment"

          "Swing Line Lender"

          "Swing Line Loan"

          "Swing Line Note"

     2.3  Section  2.22 of the  Credit  Agreement  is hereby  amended to read as
follows:

          [Intentionally left blank].

     2.4  Schedule  2.1 of the  Credit  Agreement  is hereby  amended to read as
attached hereto.

     2.5 All references in the Note to "22,200,000.00" or to "Twenty-Two Million
Two  Hundred   Thousand  and  NO/100   Dollars"  are  hereby   amended  to  read
"10,000,000.00" and "Ten Million and NO/100 Dollars," respectively.

SECTION 3.  OTHER MODIFICATIONS, RATIFICATIONS AND AGREEMENTS.

     3.1 All references to the Credit  Agreement in the other Loan Documents are
hereby amended to refer to the Credit Agreement as hereby amended.

     3.2 Borrower  hereby  reaffirms  to the Banks each of the  representations,
warranties,  covenants  and  agreements  of  Borrower  set  forth in the  Credit
Agreement,  with the same  force and  effect as if each were  separately  stated
herein and made as of the date hereof.

     3.3 Borrower hereby ratifies, reaffirms,  acknowledges, and agrees that the
Notes and the Credit  Agreement  represent  valid,  enforceable  and collectible
obligations  of  Borrower,  and that  there are no  existing  claims,  defenses,
personal or  otherwise,  or rights of setoff  whatsoever  with respect to any of
these documents or instruments.  Borrower  further  acknowledges  and represents
that no event has occurred and no condition  exists that,  after notice or lapse
of time, or both, would constitute a default under this Agreement,  the Notes or
the Credit Agreement.

     3.4 All  terms,  conditions  and  provisions  of the Credit  Agreement  are
continued in full force and effect and shall  remain  unaffected  and  unchanged
except as specifically amended hereby. The Credit Agreement,  as amended hereby,
is hereby  ratified  and  reaffirmed  by  Borrower,  and  Borrower  specifically
acknowledges the validity and enforceability thereof.

SECTION 4.  GENERAL.

     4.1 This Agreement in no way acts as a release or  relinquishment  of those
rights  securing  payment  of  the  Loans.  Such  rights  are  hereby  ratified,
confirmed, renewed and extended by Borrower in all respects.

     4.2 The modifications  contained herein shall not be binding upon the Banks
until the Administrative Agent shall have received all of the following:

          (a) An original of this Agreement fully executed by the Borrower.

          (b) Such resolutions or authorizations and such other documents as the
     Administrative  Agent  may  require  relating  to the  existence  and  good
     standing
                                      -2-
 
     of the Borrower and the authority of any person executing this Agreement or
     other documents on behalf of the Borrower.

     4.3 Borrower  shall  execute and deliver such  additional  documents and do
such  other acts as the Banks may  reasonably  require  to fully  implement  the
intent of this Agreement.

     4.4 Borrower shall pay all costs and expenses,  including,  but not limited
to,  reasonable   attorneys'  fees  incurred  by  the  Administrative  Agent  in
connection herewith, whether or not all of the conditions described in Paragraph
4.2 above are satisfied.  Banks, at their option,  but without any obligation to
do so,  may  advance  funds  to pay any such  costs  and  expenses  that are the
obligation of the Borrower,  and all such funds  advanced shall bear interest at
the highest rate provided in the Notes and shall be due and payable upon demand.

     4.5  Notwithstanding  anything to the contrary  contained  herein or in any
other instrument executed by Borrower, the Administrative Agent or the Banks, or
in any other action or conduct undertaken by Borrower,  the Administrative Agent
or the  Banks on or  before  the date  hereof,  the  agreements,  covenants  and
provisions  contained  herein shall  constitute  the only evidence of the Banks'
consent to modify the terms and provisions of the Credit Agreement. Accordingly,
no express or implied consent to any further modifications  involving any of the
matters set forth in this Agreement or otherwise shall be inferred or implied by
the  Banks'  consent to this  Agreement.  Further,  the  Banks'  consent to this
Agreement  shall not  constitute  a waiver  (either  express or  implied) of the
requirement that any further  modification of the Credit Agreement shall require
the express  written  consent of the Banks;  no such consent  (either express or
implied) has been given as of the date hereof.

     4.6 Time is hereby  declared  to be of the  essence  hereof  of the  Credit
Agreement, and Banks require, and Borrower agrees to, strict performance of each
and every covenant,  condition,  provision and agreement  hereof,  of the Credit
Agreement.

     4.7 This  Agreement  shall be binding upon,  and shall inure to the benefit
of, the parties hereto and their heirs, personal representatives, successors and
assigns.

     4.8 This  Agreement  is made for the sole  protection  and  benefit  of the
parties  hereto,  and no other  person or entity  shall have any right of action
hereon.

     4.9 This Agreement shall be governed by and construed according to the laws
of the State of Arizona.
                                      -3-

     IN WITNESS  WHEREOF,  these  presents are executed as of the date indicated
above.

                                    WELLS FARGO BANK, NATIONAL ASSOCIATION



                                    By:      /s/ Keri M. Tignini
                                    -------------------------------------------
                                    Name:    Keri M. Tignini
                                    -------------------------------------------
                                    Its:     Vice President
                                    -------------------------------------------

                                                            ADMINISTRATIVE AGENT


                                    KNIGHT TRANSPORTATION, INC.



                                    By:      /s/ Kevin P. Knight
                                    -------------------------------------------
                                    Name:    Kevin P. Knight
                                    -------------------------------------------
                                    Its:     Chairman and CEO
                                    -------------------------------------------


                                    QUAD-K LEASING, INC., an Arizona corporation



                                    By:      /s/ Timothy M. Kohl
                                    -------------------------------------------
                                    Name:    Timothy M. Kohl
                                    -------------------------------------------
                                    Its:     Treasurer
                                    -------------------------------------------

                                                                        BORROWER


                                      -4-



                              CONSENT OF THE BANKS


Re:      Knight Transportation, Inc.

         The following:

     (a) is a Bank named in that Credit  Agreement  dated April 6, 2001  between
Knight Transportation, Inc., an Arizona corporation (the "Company"), all present
and future Significant Subsidiaries of the Company (the "Borrower"), Wells Fargo
Bank,  National  Association,   as  administrative  agent  for  the  Banks  (the
"Administrative Agent"), and the Banks; and

     (b) consents to that Fifth  Modification  Agreement dated December 15, 2003
entered into between the Borrower and the Administrative Agent.

                                   WELLS FARGO BANK, NATIONAL
                                   ASSOCIATION



                                   By:      /s/ Keri M. Tignini
                                   --------------------------------------------
                                   Name:    Keri M. Tignini
                                   --------------------------------------------
                                   Its:     Vice President
                                   --------------------------------------------

                                                                          "Bank"





                                  SCHEDULE 2.1

                              COMMITMENTS OF BANKS
                               as to the Facility
                             as of December 15, 2003


                                                    
                      Bank                     %                      $
             ---------------------          -------          ------------------

          1. Wells Fargo Bank,
             National Association             100%             $10,000,000.00

             Maximum RLC
             Commitment                       100%             $10,000,000.00

Addresses:

         1.       100 West Washington
                  Commercial Banking
                  MAC S4101-251
                  Phoenix, Arizona 85003
                  Attention: Keri Tignini
                  Phone: 602-378-4593
                  Fax: 602-378-4758