Abstract: Ultrasonic irradiation technique was firstly applied to a membrane distillation system to increase the distillation rate in the paper.The effect of ultrasonic parameters on the mass and heat transfer,and the ultrasonic mechanismwas studied by use of continuous and intermittent ultrasonic irradiation.The results indicate that the distillation rateincreases with the increase of ultrasonic power and the 30% of the distillation rate can be increased in the experiments.Moreover the effect of the continuous irradiation is better than the intermittent one.As for ultrasonic mechanism it isshown that ultrasonic cavitation,acoustic streaming and membrane vibration improve the temperature-concentrationpolarization and clean the membrane surface,resulting in the increase of distillation rate.Here the acoustic streamingand cavitation,which improve the temperature-concentration polarization 3 are more important mechanism.Finallythe theoretical formula for calculation of distillation rate in reference 10 was proved also valid in the case of ultrasonicirradiation in the paper.