Abstract: This study presents a multi-axial fatigue model to predict fatigue lifetime of involute spur gear considering the elastohydrodynamic lubrication conditions and subsurface stress distributions. The proposed model considered the effect of both the stress history of certain point under tooth surface and material properties, and obtained the fatigue lifetime distribution of entire gear contacts. Firstly, the finite line-contact elastohydrodynamic lubrication model of the involute spur gear was established and the pressure distribution, oil film thickness and subsurface stress were obtained. Then, by analyzing the relationship between the movement of computational domain and the process of gear contact, the stress history of certain point in tooth was obtained. Finally, the gear fatigue lifetime of complete meshing process were predicted with multi-axial fatigue model based on the stress history. The results show that tooth surface roughness had a significant effect on gear fatigue lifetime. With the increase of the amplitude of surface roughness, subsurface maximum stress moved to the teeth surface, which resulted in the development of low fatigue lifetime region to the teeth surface and further extension to the entire single tooth meshing area; however, it had little effect on gear fatigue lifetime if the surface roughness reduced to a certain extent.