To study the nonlinear dynamic behavior of a rolling bearingJeffcott rotor system, the governing differential equations of motion of the system are established and solved by RungeKuttaFelhberg algorithm. The nonlinear dynami c behavior of the system are illustrated by means of bifurcation diagrams, Poinc arｅmaps and frequency spectrum diagrams. Numerical results show that peri odic re sponse with various frequencies, including external forcing frequency, param etrical forcing frequency, or the linear combinations of them and chaotic respon se may exist. It is also shown that increase of radial internal clearance may en hance nonlinearity of the system.