The effect of ultrasonic viscosity reduction technology on the viscosity and temperature of lubricating oil was studied.The relationship between the viscosity and temperature of lubricating oil under the action of ultrasound and water bath was studied,and the mechanism of ultrasonic viscosity reduction was analyzed.The results show that,under the same temperature point,the effect of ultrasonic viscosity reduction is better than that of water bath viscosity reduction,and the viscosity reduction of ultrasonic viscosity reduction is greater than that of water bath heating.The viscosity reduction rate of lubricating oil is increased first and then tended to be stable with the increasing of ultrasonic action time.The viscosity reduction rate of lubricating oil 10W-50 is 741% under the ultrasonic action of 42 kHz and 70 W for about 30 min.The ultrasonic viscosity reduction mechanism of lubricating oil includes ultrasonic cavitation and mechanical effect besides the thermal effect of ultrasound.At the same temperature,the thermal effect of ultrasound on viscosity reduction of highviscosity oil samples is better than that of lowviscosity oil samples.The mechanical effect of ultrasound could play the role of stirring and homogenization,which can promote the homogenization of lubricating oil temperature.