The resignation was effective Dec. 21, according to a filing Tuesday with the U.S. Securities and Exchange Commission. The filing provided no other details on the resignation.
PeopleSoft and Oracle declined to comment on the filing, and Duffield could not immediately be reached for comment.
former CEO, PeopleSoft
Earlier this month, PeopleSoft's boardby Oracle after more than 18 months of hostility between the two companies. Oracle executives had said they expected to complete the merger by Thursday.
Duffield, who founded Pleasanton, Calif.-based PeopleSoft in 1987, was appointed CEO in October afteras president and chief executive officer. A PeopleSoft board member later testified that Conway was fired in large part to Wall Street analysts when informing them last year that Oracle's offer to buy the company was no longer a disruptive influence.
At the time, Duffield said he was looking forward to returning to the helm of PeopleSoft.
"It's great to be back," Duffield said. "It's an exciting day for me, personally. I look forward to coming back to work with a strong management team."
He also said that he didn't see his new stint as CEO as an interim role. "I'm here for the long term. I'm energized, and I can't imagine a better team to work with."
After leading the company he founded for 12 years, Duffield in 1999, handing the post to Conway. PeopleSoft was struggling financially at the time Duffield resigned. Conway had as president and chief operating officer.
The hostile takeover bid by Oracle for PeopleSoft was marked by mudslinging and accusations between the two business software makers. The events included afor defamation over information released by Oracle about Duffield's stock sales.
In a letter to Ellison, Duffield defended his stock sales last fall--a period when PeopleSoft issuedto Wall Street about its quarterly performance.
In September 2003,, who was PeopleSoft's chief executive at the time, told analysts during a conference that Oracle's hostile bid was not affecting the company's sales. PeopleSoft's board, however, was aware of his misstatements and soon issued a revised filing with the SEC.
Also that month, Duffield sold 400,000 shares. Then, in October, the number of shares he sold jumped to 776,469. His stock sales later fell back to 100,000 in November and rose again to 440,050 in December.
Duffield defended his actions, noting that the sales took place through a, as well as through a charitable foundation for animals.
Duffield concluded his letter by saying he would consider taking legal action if Oracle continued in the same vein.
Oracle estimated months ago that it would slash 6,000 jobs to wring costs out of the merger, but the company has yet to discuss its official lay-off plans.
CNET News.com's Dawn Kawamoto contributed to this report.