The frictional moments are calculated respectively based on the Hertzian contact theory and the method of effective ball considering elastic hysteresis of material, spinning sliding of roller and viscosity of lubricating oil, and the influences of contact angle, helix angle and tooth number of roller thread on frictional moment are analyzed. Furthermore, a calculating model of transmission efficiency for planetary roller screw is developed and the relationships of between the screw rotational speed, structure parameters mentioned above and transmission efficiency are discussed. The results show that the frictional moment is principally generated by roller's spinning sliding. Increasing the rotational speed of screw leads to decreasing of the transmission efficiency and a higher axial load can increase the transmission efficiency at the same rotational speed of screw. A higher contact angle can decrease the frictional moment and increase the transmission efficiency. The helix angle has less influence on frictional moment and can improve the transmission efficiency. Besides, it is helpful to enhance the carrying capacity and reduce the total frictional moment by increasing the tooth number of the roller thread.