The existing method of the fatigue strength check of gear is mainly based on the maximum Hertz stress under dry contact conditions. However, gears always operate under lubricant conditions, and meanwhile subject to the time-dependent stress which below the tooth surface. Combining the geometry and kinematics parameters of gear as well as the lubricant characteristics, a gear contact analysis model of elasto-hydrodynamic lubrication (EHL) is established considering the transient variables, i.e., the curvature radius, the entrainment velocity and the mesh force. The sub-surface stress field solved by DC-FFT algorithm is combined with the critical plane criterion to study the fatigue life and creak initiation during the gear engagement,and to discuss the influence of work condition on the fatigue life.The results show that the fatigue life under EHL is higher than that in the Hertz dry contact.This is due to the change of critical plane angle affected by stress field and pressure, which influences the fatigue parameter and fatigue life. And the fatigue life significantly decreases in nonlinear with the increasing input-torque, thus keeping the position of crack initiation away from the tooth surface. The fatigue life hardly changes with the increasing speed under Newtonian isothermal fluid.