The choice of lubricant for gearbox will affect not only the contact strength of gears，but also the transmission efficiency.Taking the kinematic viscosity index as the object,a lubricating oil selection method was proposed based on the analysis of contact strength and churning torque loss of gears,which can minimize the torque loss with adequate strength.The chosen lubricating oil from this method was verified during the development of a transmission project.The results show that,with the chosen oil,the contact strength of the gears meets the requirement and the transmission efficiency of the gearbox is improved,indicating that the proposed method is plausible and helpful to the engineering practice of lubricant selection.