Abstract: A spur face gear mechanical efficiency model based on meshing simulation and elastic hydrodynamic lubrication theory was introduced. The impact of the input torque and input speed on the spur face gear mechanical efficiency was revealed. The model mainly included the numerical simulation of meshing process which was based on Tooth Contact Analysis(TCA) and Loaded Tooth Contact Analysis(LTCA) and the coefficient of sliding friction model which was based on non-Newtonian quasi steady state thermal elastic hydrodynamic lubrication theory lubrication. The calculation of model shows that the coefficient of sliding friction had a significant effect on the meshing efficiency. The sliding friction coefficients of different tooth face positions were different. The sliding friction coefficient depended on the input torque and input speed.