"Donald is out of control," Stewart told TV's "Access Hollywood."
"By the time he's finished, he will have fired everyone around him and there will be nobody left," she said. "Poor Donald will be sitting there on his little pedestal all by himself."
Stewart tried to challenge Trump with a TV show similar to "The Apprentice" in 2005. She told Fortune magazine then that she expected to fire Trump, People reported.
Instead, her own show was canceled in November.
In February, Trump drove the nail into her coffin in an open letter, saying he was considering producing a syndicated daytime show called "The Boardroom" to destroy her in that time slot too.
"I almost can't believe my longtime friend Donald Trump wrote it," Stewart told People at the time.