With the in-depth researches on MEMS sensors, a lot of mechanical problems are encountered inevitably. Especially for the comb capacitive micro-machined gyroscope, a typical MEMS inertia sensor, the speciality of its structure and micro size determines the characteristics of the mechanical environment. With the reduction of structural size, the ratio of surface force and body force has increased clearly. Comparing with the roles of body force, the roles of surface force in MEMS sensors become more and more important. The micro forces, such as electrostatic force, Van de Waals force, capillary force and air-damping force were analyzed, and the action extent of these micro forces were obtained.