EXHIBIT 5.1

                        [GOODWIN PROCTER LLP LETTERHEAD]

                               September 15, 2004

Moldflow Corporation
430 Boston Post Road
Wayland, Massachusetts 01778

Re:   Registration Statement on Form S-8
      Registration Statement No. 333-100122

Ladies and Gentlemen:

      We have acted as counsel for Moldflow Corporation, a Delaware corporation
(the "Company"), in connection with the Registration Statement on Form S-8
(Registration Statement No. 333-100122) (as amended from time to time, the
"Registration Statement") filed with the Securities and Exchange Commission
pursuant to the Securities Act of 1933, as amended (the "Securities Act"). The
Registration Statement relates to the registration and issuance by the Company
of up to 7,354 additional shares of common stock, par value $0.01 per share, of
the Company (the "Shares") which Shares have been authorized for issuance
pursuant to the Moldflow Corporation 2000 Stock Option and Incentive Plan (the
"Plan").

      In connection with rendering this opinion, we have examined (i) the Third
Amended and Restated Certificate of Incorporation of the Company, as amended to
date and as on file with the Secretary of State of the State of Delaware, (ii)
the Second Amended and Restated By-laws of the Company, (iii) such records of
the corporate proceedings of the Company as we deemed material, (iv) the
Registration Statement and the exhibits thereto, (v) certificates of public
officials, and (vi) such other certificates, receipts, records and documents as
we considered necessary for the purposes of this opinion. In our examination, we
have assumed the genuineness of all signatures, the legal capacity of natural
persons, the authenticity of all documents submitted to us as certified,
photostatic or facsimile copies, the authenticity of the originals of such
copies and the authenticity of telephonic confirmations of public officials and
others. As to facts material to our opinion, we have relied, without independent
verification, upon certificates or telephonic confirmations of public officials
and certificates, documents, statements and other information of the Company or
representatives or officers thereof.

      We are attorneys admitted to practice in The Commonwealth of
Massachusetts. We express no opinion concerning the laws of any jurisdiction
other than the laws of the United States of America, The Commonwealth of
Massachusetts, and the Delaware General Corporation Law, and we also express no
opinion with respect to the blue sky or securities laws of any state, including,
without limitation, Massachusetts or Delaware.

      Based on the foregoing, we are of the opinion that the Shares covered by
the Registration Statement will be, when and if issued against payment therefor
in accordance with the terms of the Plan and any agreement thereunder, validly
issued, fully paid and nonassessable under the Delaware General Corporation Law.

      The foregoing assumes that all requisite steps will be taken to comply
with the requirements of the Securities Act and applicable requirements of state
laws regulating the offer and sale of securities and applicable requirements of
The Nasdaq National Market.

      This opinion shall be interpreted in accordance with the Legal Opinion
Principles issued by the Committee on Legal Opinions of the American Bar
Association's Business Law Section as published in 53 Business Lawyer 831 (May
1998).

      This opinion is being furnished to you solely for your benefit in
connection with the Registration Statement, and may not be used or relied upon
by you for any other purpose, nor may this opinion be quoted from, circulated,
relied upon or otherwise referred to, by any other person or entity without our
prior written consent. This opinion is given as of the date first set forth
above, and we assume no obligation to update this opinion.

      We hereby consent to being named as counsel to the Company in the
Registration Statement and to the inclusion of this opinion as an exhibit to the
Registration Statement. In giving such consent, we do not thereby admit that we
are in the category of persons whose consent is required under Section 7 of the
Securities Act or the rules and regulations promulgated thereunder by the
Securities and Exchange Commission.

                                             Very truly yours,


                                             /s/ GOODWIN PROCTER LLP

                                             GOODWIN PROCTER LLP