He said on Monday that he won't attend the debate unless he thinks he will be treated fairly.
"If I think I’m going to be treated unfairly, I’ll do something else. But I don’t think she can treat me fairly, actually. I think she’s very biased," he told CNN. "But that doesn't mean I don't do the debate. I like doing the debates. I've won every single debate according to every poll."
Trump on Saturday said that Kelly should not be allowed to moderate the debate because she is is biased.
During his Monday interview on CNN, Trump also credited himself with Kelly's success as a Fox News host.
"I might be the best thing that ever happened to her," he said. "Who ever even heard of her before the last debate?"
In a Monday statement, Fox News brushed off Trump's threats to boycott the debate.
"Sooner or later Donald Trump, even if he's president, is going to have to learn that he doesn't get to pick the journalists -- we're very surprised he's willing to show that much fear about being questioned by Megyn Kelly," the network said, according to CNN.
Watch part of Trump's interview on CNN: