Is a polar bear stronger than a grizzly bear?

Yes, because a polar bear is much larger and powerful in size: Polar bears weigh between 600 and 1200 lbs, whereas grizzly bears average between 500 and 800 lbs. Polar bears can hunt and kill large walruses and even drag a dead beluga out of the water onto the ice while grizzly bears have only few difficulties with killing bison, especially cows and young calves. They do have a much higher difficulty killing bull bison, though.