A modified fractal model of static friction coefficient of joint interfaces based on normal load model and modified size distribution is proposed. Furthermore, numerical simulation was carried out to obtain the complicated nonlinear relations. The results showed that the static friction coefficient versus the micro convex nonlinear relation with the normal load and increased with the normal load and material property increase, decreased with fractal characteristic length scale parameter G*, but complicatedly with D. This is explained that the modified model could be used to analyze and predict static friction coefficient.