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For the purpose of improving the efficiency of automotive transmission, there is a greater need to modify even the gear dimensions and tooth surface for reducing gear mesh power loss. In our study, we developed the prediction program and examined some reduction methods of the mesh power loss. To predict the mesh power loss of transmission gears accurately, it is necessary to estimate the friction coefficient at tooth meshing surface. But there was no formula with high accuracy for useful estimation of friction coefficient under mixed lubrication condition such as gear meshing condition. The one of authors proposed the new estimation formula of friction coefficient. In this paper, the usefulness of proposed estimation formula is confirmed and the prediction program of mesh power loss applying the proposed estimation formula of friction coefficient is also validated by several gear pair tests. From this prediction program and some experiments, the influence of gear dimensions, tooth surface modification, tooth surface roughness and driving condition on mesh power loss is discussed.