Abstract： Based on the contact mechanics theory, the deformation behavior of a single contacting asperity was studied, a micro contact model was  proposed considering elastic-plastic transition deformation mechanism of rough surface joints, and a mathematical model of normal contact load and contact stiffness were established. Besides, the fractal model of mechanical joints normal contact load and contact stiffness were presented based on the fractal geometry theory. The results show that the plasticity index is lesser, the relationship between normal contact load and contact stiffness is approximately linear, and with the plastic index increasing, the relationship between normal contact load and contact stiffness is strongly nonlinear. The normal contact stiffness increases with increasing fractal dimension and normal contact load and decreases with increasing dimensionless fractal scale parameters. Moreover, the joints normal contact stiffness whose machining surface were milled and ground were calculated respectively by using the present model. The comparion of test results show that the present model is consistent with the experiment result.