An ultrasonic irradiation technique was applied to an air gap membrane distillation (AGMD) system to enhance the permeability for the membrane distillation of various aqueous solutions. In this study, an ultrasonic stimulation of resonance frequency of 20 kHz and irradiation power up to 90 W was applied to a flat-plate AGMD system of 1 mu m PTFE membrane with a membrane distillation temperature difference up to 55 degrees C. The feed solutions tested include tap water and various sodium chloride solutions. Permeate mass flow rate, heating and cooling temperatures, irradiation power and acoustic intensity in feed solutions are directly measured. It was found that the permeate flow rate with the ultrasonic stimulation increases up to 25% than that without ultrasonic irradiation The permeate how rate also increases with an increase of irradiation power. Experimental investigation of basic mechanisms pertaining to the ultrasonic enhancement of AGMD was also attempted via both intermittent ultrasonic irradiation and study of the effect of ultrasonic irradiation on surface temperature of membrane. The results suggest that, for a continuous ultrasonic stimulation, the domination mechanisms are microstreaming and cavitation. The significant reduction of temperature polarization is validated by both temperature measurements and ultrasonic intensity measurements.