FactSet Research Systems (FDS) 10-K 2008 FY Annual report

EXHIBIT 32.1

CERTIFICATION OF CHIEF EXECUTIVE OFFICER PURSUANT TO

18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Annual Report of FactSet Research Systems Inc. (the “Company”) on Form 10-K for the period ending August 31, 2008, as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Philip A. Hadley, certify, pursuant to 18 U.S.C. §1350, as adopted pursuant to §906 of the Sarbanes-Oxley Act of 2002, that:

(1) The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2) The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

/s/ PHILIP A. HADLEY                                        

Philip A. Hadley

Chairman and Chief Executive Officer

(Principal Executive Officer)

October 30, 2008