A new viewpoint about the cause of earthquakes is proposed in this paper after investigation into the relationship between the earth rotation and the M8.0 earthquake on May 12th , 2008 in Wenchuan, Sichuan Province. The fundamental cause of earthquakes could lie in the variation in velocity of the earth rotation. The lithosphere of the earth consists of plates with different volumes and masses. For instance, the oceanic plate is thinner and lighter, and the continental plate thicker and weightier. The variation in velocity of the earth rotation could result in the motional difference between them, which could make the plates collide or rub, and cause earthquakes.