Rotor misalignment is one of the most important faults in rotating machinery, which can also cause excessive vibrations and dislikeful noises. Under some assumptions,such as rigid rotors and small angular misalignment, the dynamics model of rotor system is developed after considering the misalignment of roto r s and the unbalance of mass. Firstly, motion equations are der ived using the Lagrange′s approach, and it presents that angularly misaligned rotor system is a strongly nonlinear one with parametric excitations. Based on the har monic balance method, the nonlinear dynamics characteric of the system is analyzed. The results show that the misaligned system exists the same component of steady state responses at rotating frequency as the unbalanced system presents , and exists the combinational frequencies composed of the rota ting frequency and the frequency in the angular direction, and its amplitudes of responses at combinational frequencies are depended on the angular misalignment and other parameters of the system. Finally, the nonlinear dynamics characterist ics of rotor system with angular misalignment are analyzed by the numerical method.