Sheehan last month said she intended to run against Pelosi, the House speaker, if the San Francisco congresswoman didn't move to impeach President Bush by July 23.
Sheehan said Thursday that Pelosi had "protected the status quo" of the corporate elite and had lost touch with people in her district, most of whom want American troops out of Iraq.
Calls to Pelosi's office seeking comment were not immediately returned.
After her 24-year-old son, Casey, was killed in Iraq in 2004, Sheehan gained international attention by setting up camp outside the president's Texas ranch and demanding to meet with him.
On Thursday, a tearful Sheehan said her son inspired her to run against Pelosi, who has represented San Francisco in Congress since 1987.
"The country is ripe for a change," said Sheehan, who spoke at a podium with her son's photograph attached to it. "It's going to start right here and right now."
Sheehan said she would run as an independent on a platform of universal health care, making college affordable and improving ethics in the legislative and executive branches. She did not offer specifics.
The money being spent on the Iraq war should be used to help the country's shrinking middle class, she said.
Sheehan said she had no funds for a campaign but planned to immediately start raising money. She said she wouldn't accept money from corporations.
Daniel Ellsberg, the former high-level Pentagon analyst who in 1969 leaked the Pentagon Papers to Congress and the media and a Sheehan supporter, said Sheehan has a difficult task.
"At the moment, facing a well-funded, powerful incumbent without party support, the odds against Cindy appear insuperable," said Ellsberg, who was arrested twice with other protesters outside Bush's Texas ranch.
"But we plan to change that."