In this paper,the existing problems with the frictional coefficient in the classical friction theory are analyzed. In order to solve these problems,a frictional micro volume is established based on the theories of elasticity and plasticity,and three assumptions are thus proposed as follows: ( 1) the friction is proportional to the real contact area; ( 2) the frictional shear stress is independent of the normal stress; and ( 3) there is no abrasion or wear. Then,the friction model is constructed based on the real contact area and is employed to analyze the friction problems with the equal-height roughness and the random roughness as well as under extreme conditions. The results show that the proposed model is significantly different from the classical friction coefficient method; and that it reflects the origin of interfacial friction and can be used to effectively solve different friction problems.