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 I, Kenneth L. Schroeder, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that the Annual Report of KLA-Tencor Corporation on Form 10-K/A for the fiscal year ended June 30, 2003 fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 and that information contained in such Annual Report on Form 10-K/A fairly presents in all material respects the financial condition and results of operations of KLA-Tencor Corporation. September 29, 2003 By: /s/ Kenneth L. Schroeder Dated Name: Kenneth L. Schroeder Title: Chief Executive Officer CERTIFICATION OF CHIEF FINANCIAL OFFICER PURSUANT TO 18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002 I, John H. Kispert, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that the Annual Report of KLA-Tencor Corporation on Form 10-K/A for the fiscal year ended June 30, 2003 fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934 and that information contained in such Annual Report on Form 10-K/A fairly presents in all material respects the financial condition and results of operations of KLA-Tencor Corporation. September 29, 2003 By: /s/ John H. Kispert Dated Name: John H. Kispert Title: Chief Financial Officer A signed original of this written statement required by Section 906 has been provided to KLA-Tencor Corporation and will be retained by KLA-Tencor Corporation and furnished to the Securities and Exchange Commission or its staff upon request.