The Elastohydrodynamic lubrication (EHL) model of the involute helical gears was developed by simplifing the contact between tooth surfaces in the mesh of a pair of helical gears into a finite line-contacts lubrication problem. The time-varying contact parameters of helical gears such as the variations of radius of curvature, the tangential surface velocities and the loads in the mesh were considered in the analysis model. Based on the unified Reynolds equation approach, the transient solutions of the entire meshing process were obtained for a pair of teeth. The results show that the distributions of pressure and film thickness varied greatly at different meshing positions. The film thickness was affected significantly by the equivalent radius of curvature of the contacting tooth surfaces at every meshing point. Transient effect was relatively weak for helical gear. The lubricating conditions seemed to be poor at the positions near the root and the pitch of the pinion. It will be helpful to improve the gears lubrication by increasing the pressure angle.