Fans of both shows had hoped Gellar would make an appearance as "Angel" ends its fifth and final season this May.
"Buffy" ended its own six-year run last spring. David Boreanaz, who first played Angel the vampire with a soul and Buffy's brooding first love, made a guest appearance on the series shortly before it ended and helped Buffy and her current beau, Spike, played by James Marsters, save the world.
However, Gellar is too busy with her film career to return the favor, disappointing fans hoping for a resolution to the Buffy-Angel-Spike love triangle, TV Guide Online said Thursday.
Spike, also a vampire with a soul, moved to "Angel" when "Buffy" ended last year.
Gellar was invited to return for the second-to-last episode, but was working in Tokyo when it was shot, the entertainment magazine noted.
She was free to appear in the final episode, but executive producer Joss Whedon said he didn't want it to "revolve around a guest star."
"We will deal with the issue of Buffy and how much she means to Angel and Spike, but I want to end the show with the people who've been in the trenches together, the characters who have lived -- and occasionally died -- together ... the regulars," he said.