A mathematical model of a rotor with nonlinear oil��film force was presented, in which coupled bending and torsion were considered. The unbalance response of the rotor was numerically calculated by Rugge-Kutta method, and the bifurcation diagrams and waterfall plot were obtained to analyze dynamic characteristics of the system. The results reveale multiform complex nonlinear dynamic behavior of the rotor, that comprises periodic, double periodic and quasi��periodic motion. By comparing with dynamic characteristic of the system without considering the interaction of bending and torsion, it shows that bending vibration is heavily affected by the interaction of bending and torsion when rotating speed is near by the critical speed.The numerical study illustrated characteristics of torsion with increasing rotating speed. The results can provide a deeper understanding to coupled bending and torsion of the rotor��bearing system.