On the basis of fractal model of tangential contact stiffness of joint surfaces, and combining with the fractal model of two cylinders’ contact stress, calculating fractal model of tangential contact stiffness of gear joint surfaces was set up. This model synthetically considered the microcosmic characters and macroscopical factors of gear joint surfaces, and the results by the model showed no relationship with the resolving ability of the instruments and dimension independency. Simulation results indicated that the tangential contact stiffness was proportional to fractal dimension, material properties parameters and the number of gear teeth, and inversely proportional to roughness amplitude.