Abstract: In most cases, people considered the coefficient of friction as constant in the calculated efficiency formulae in gear drives and usually neglected the effect due to the differences between the driver and the follower on the efficiency calculation. These formulae may be used for low and medium powered gear drives, but caused rather bigger errors on the efficiency calculation of powerful gear drives and KHV planetary gear drives. The authors suggested that the coefficient of friction Ft may be related to the position of meshing point. They considered it as the. Factor of friction, investigated the effect of the driver and the follower on efficiency and gave out seperately the accurate formula and the approximation formula on the efficiency calculation. The formulae may give some reference to the designer on gear drives.