Abstract: Considering the strong nonlinear oil-film force and influence of nonlinear friction force, a dynamic model was set up for the rotor-bearing system at rub-impact faults. The nonlinear dynamic behaviors in running were investigated widely on a basis of modern nonlinear dynamics and rotor dynamics theory with the influence of the coefficient of relative speed between rotary and stationary parts on the bifurcation and chaotic motion of the system discussed. Lots of nonlinear phenomena investigated revealed that within the subcritical speed range both the quasi-period and chaotic motion regions increase with the increase of relative speed coefficient. However, within the supercritical speed range, the chaotic motion region decreases, but the rub-impact forces increase and the quasi-period motions become gradually the period 3 ones. These provide a theoretic basis reference for the failure diagnosis to the rotor-bearing systems at rub-impact faults.