Exhibit 32.1

RITA Medical Systems, Inc.

CERTIFICATION 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 Quarterly Report of RITA Medical Systems, Inc. (the “Company”) on Form 10-Q for the three and six months ended June 30, 2003, as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Donald Stewart, Acting Chief Executive Officer, Chief Financial Officer and Vice President, Finance and Administration of the Company, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 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 result of operations of the Company.

/S/    DONALD STEWART        

Donald Stewart

Acting Chief Executive Officer,

Chief Financial Officer and

Vice President, Finance and Administration

August 13, 2003