In order to investigate the effect of tooth friction on the transmission efficiency of high power density gear,a method to predict the gear transmission efficiency and to analyze its dynamic characteristic is proposed onthe basis of virtual prototype technology.In this method,a flexible multi- body dynamic model of gear,rotation shaftand bearing is built when the flexibile deformation of gear and rotation shaft as well as the time- varying mesh stiff-ness of gear are taken into account.Then,the dynamic characteristics and the efficiency of gear transmission sys-tem are obtained by means of dynamic equations of prototype,and the effects of the tooth friction coefficient on theefficiencies and the dynamic characteristics of spur,helical and double helical gear transmission systems are alsoanalyzed.The results show that both the contact stress and the dynamic tangential friction force increase when thetooth friction coefficient increases,and that,at a high rotation speed,the contact stress of double helical gear is theleast in comparison with those of helical and spur gears,and its efficiency is higher than that of other gears.