In this work, effect of ultrasonic waves on permeation flux of a microfiltration membrane fouled with fresh cow milk as feed was studied. Also, effects of various parameters such as feed pressure, ultrasonic irradiation power, ultrasonic source diameter, distance between the irradiation source and the fouled membrane surface and pulse irradiation were studied and permeation flux variations were recorded. In all experiments, it was found that ultrasonic irradiation increases the microfiltration flux. The highest flux enhancement factor (%FE = 490%) was obtained at a pressure of 0.5 bar, power of 40W and distance of 2.6 cm. Continuous irradiation of ultrasonic waves uplifted the flux enhancement factor in a degree of 33% more than pulse irradiation. Also, both SEM analysis and permeation flux of pure water confirmed that ultrasonic irradiation has no destructive effects on the membrane surface. (C) 2011 Elsevier Ltd. All rights reserved.