In order to calculate the nonlinear oil force of each pad of tiltingp a d journal bearing in a single pad coordinate, the variational 〖ＨＪ２／５〗co nstraint approac h is used in this paper. According to the physical characteristic of the oil fil m, an isoparametric finite element with eight nodes method is adopted to convert the Reynolds equation continuously to the discrete form of finite dimensional a lgebraic variational equations satisfying certain constraint conditions at each step of the dynamic integration and iteration. The oil force of fivepad tiltin g pad journal bearings is obtained by the assembly method. In order to analyze t h e unbalanced response behavior of the symmetrical rigid rotor dynamic system sup ported by the fivepad tiltingpad journal bearing, the RungeKutta method an d a Poincare map are used. The numerical results show complex nonlinear behaviors o f the system, such as periodic, perioddoubling, chaos, and etc.