Dos Santos (15-1), who scored a 64-second knockout victory in their first meeting in November 2011, came in at 239 pounds. Velasquez (10-1) weighed in at 240.
The two heavyweights have been respectful of one another in the buildup to UFC 155, but that hasn't stopped the champ from predicting a repeat knockout performance at some point before the end of the third round.
"Cain Velasquez, for sure, is a great opponent," Dos Santos said. "But I know how (the fans) like knockouts. So, one more is coming for you."
All 24 fighters set to compete at UFC 155 made weight. In the co-main event, Jim Miller (21-4) will look to finish 2012 on a high note with a win over the surging Joe Lauzon (22-7). Lauzon was originally scheduled to face Gray Maynard, but that fight fell apart because of injury. Both Lauzon and Miller weighed in at 156 pounds.
The fight was scheduled to take place earlier this month at "The Ultimate Fighter" Finale, but was canceled at the last moment when Varner became ill backstage.