Abstract: Finite element (FE) model of transmissions first gear pair was built. Based on explicit dynamic theory, transient meshing process of the gear pair under maximum engine output torque was simulated. At the critical spots of the gear teeths surfaces and roots, the amplitudes of main principle stresses and equivalent stresses, as well as their changing patterns were obtained. On the basis of the FE analysis model, the investigation was conducted to find out the influencing weights and changing rules of some important factors on the loading conditions of the gear pair, such as stiffness of gear shaft, assembling error, as well as tooth root fillet radius, etc. The results and conclusions from this research work can provide essential information for structural scheme modification to achieve higher strength and reliability, as well as to determine the processing and assembling parameters.