Abstract: The no -load speed of ultrasonic motor ( USM ) with different friction materials of four selected engineering plastics was measured by using testing system of friction drive characteristics of USM under vacuum and low temperature The effects of vacuum degree , environmental temperature and friction materials on no -load properties of traveling wave ultrasonic motor were researched The results show that the no -load speed of ultrasonic motor decreased with the increase of vacuum degree and decrease of environmental temperature The friction materials which is suitable for vacuum and low temperature was found by comparing the drive properties of the four engineering plastics