The German team have scored two second places in succession and find themselves second in the race for the constructors' crown after the opening two grands prix, splitting leaders McLaren and third-placed Ferrari.
"BMW here and there will have a few highlights, maybe more than we think, but calculating all things, I believe, no, they cannot sustain it over the season,"
Schumacher told Reuters in an interview in Brussels, where he was spearheading a campaign to reduce the number of drink-driving deaths across the world.
The 39-year-old added that he did not see any other team competing with last season's top teams.
"I believe it is a straight fight between Ferrari and McLaren," he said. "There will be no other team fighting realistically towards the end of the championship."
Ferrari won the constructors' championship last year after McLaren were stripped of all their points for a spying scandal.
Schumacher said he did not see BMW drivers Nick Heidfeld or Robert Kubica challenging for the drivers' title but he had some words of encouragement for fellow German Sebastian Vettel, despite two retirements so far for the Toro Rosso team.
"I think my German teammate Sebastian Vettel particularly will be a lot of a surprise this season," Schumacher said.
"He is going very well despite the results. He went very well in Australia and Malaysia until retirement. In this respect I think he will be the surprise of the season."
autosport.com