Abstract: A crack initiation model was built for helical gear based on the risk accumulation theory considering influences of the tribo-dyanmic characteristics on the contact pressure under mixed EHL interface. In the modeling of short crack propagation, the non-linear and discontinuous characteristics were taken into account. Besides, a unified equation for long crack propagation covering various cracks with different scales was proposed to predict the whole fatigue life of gear pair. The results show that the contact pressure was significantly influenced by the dynamic meshing force and mixed lubricating performance. Several peaks of the effective shear stress appeared in subsurface due to the residual stress, which led to different contact failure forms after numbers of periodical meshing impacts.