The effect of low power ultrasonic radiation on anaerobic biodegradability of sewage sludge was investigated. For this purpose, soluble substances and variation of microbial system of sewage sludge subjected to low power ultrasonic radiation were tested. The well known hydromechanical shear forces and heating effect of low frequency ultrasound plays a major role in the sludge pre treatment process. More, the increase of soluble substance may partly result from the destruction of microbial cell by excess ultrasonic pretreatment, which will inhibit the anaerobic process. By orthogonal tests, the optimal parameters were found to be an exposure time of 15 min, ultrasonic intensity of 0.35 W/cm(2) and ultrasonic power density of 0.25 W/ml. Under the optimal condition, anaerobic biodegradability of sewage sludge (R((vss/ss)) %) was increased by 67.6%. Consequently, it can be concluded that low power ultrasonic pretreatment is a valid method for improving anaerobic biodegradability of sewage sludge. (C) 2009 Elsevier Ltd. All rights reserved.