Abstract: The classical theory of gear contact fatigue strength is based on the Hertz contact theory with the assumptions that teeth surfaces are ideally smooth without lubrication. However, the gear pairs often operated in mixed lubrication and the teeth surfaces were rough actually. This paper aims to establish a contact fatigue model for involute helical gear, which was based on the analysis of the gear contact under mixed lubrication. The model consisted of two submodels. Firstly, finite line contact elastohydrodynamic lubrication (EHL) model of the involute helical gears was developed by assuming each meshing instant as the contact between two tapered rollers in opposite orientation. The influences of the load distribution along the line of action, the length of the line of contact and the entrainment velocity on the lubrication were taken into account. The pressure distribution and the oil film thickness in the entire computational domain were obtained based on a unified Reynolds equation system approach. Secondly, the subsurface von Mises stress field was calculated with the pressure distribution from the mixed-EHL analysis, and then the contact fatigue life of the gear pairs was predicted by using the fatigue life model developed by Zaretsky. Finally, the developed model was applied to study the effect of gear surface roughness and lubricant viscosity on gear fatigue life. The results show that both of them had a significant influence on the contact fatigue life of gear.