In the present paper, two different methodologies for simulate the dynamic behavior of gears are described and compared. The nonlinear torsion vibrations of geared systems are studied at resonance frequency with the contact loss of the meshing teeth. In the proposed model a time-varying mesh stiffness of tooth pairs and a viscous damping proportional to the meshing stiffness are considered. Gear errors of each meshing tooth pair are also included. Dynamic motion of the spur gear pair over wide range of excitation frequency using the finite element method was simulated. Results of bending stress distribution during overall operation speeds were obtained by FEA. It may serve as a tool for aiding the gear fault diagnosis.