"Senator Obama has made a choice more out of weakness than strength," the former New York mayor said Sunday in an appearance on ABC's "This Week" program.
Biden, a Delaware senator, is chairman of the Senate Foreign Relations Committee. Some analysts said he bolsters the Democratic ticket's foreign policy credentials.
Giuliani said the "strong pick" for Obama would have been Sen. Hillary Clinton of New York senator lost to Obama in the protracted Democratic primary.
"The strong choice would have been Hillary Clinton, the obvious choice would have been Hillary Clinton," Giuliani said. "She had 50 percent of the Democratic vote. Obama has 50 percent of the Democratic vote, you almost have to go to extraordinary lengths to avoid her as the vice presidential pick of the party."
Obama's chief campaign strategist David Axelrod said Obama picked Biden because he felt the Delaware senator was "the best partner" for him.
"Joe Biden is a guy who, obviously, is accomplished and knowledgeable," Axelrod said. "He's an independent guy who will tell the president what he needs to know even if he doesn't want to hear it. And that's what you want in a vice president."