Ultrasonic technology is successfully applied to enhance the permeate flux of membrane distillation. In this study, two major mechanisms of ultrasonic enhancement on membrane distillation, ultrasonic cavitation and acoustic streaming, are formulated in an analytical modeling. Ultrasonic induced membrane vibration, acoustic energy dissipation in viscous fluid, and membrane absorption and transmission of acoustic energy are also included in the modeling approach. Based on this model, effects of ultrasonic intensity, ultrasonic frequency, solution temperature and excited membrane area on permeate flux of membrane distillation are theoretically analyzed. It is found that the enhancement ratio can be improved by increasing ultrasonic intensity, decreasing ultrasonic frequency or decreasing the solution temperature. The current study also demonstrates that an enhancement up to 200% can be reached with an ultrasonic intensity from 0 to 5 W/cm(2).