Ultrasound was applied to the control of membrane fouling development on-line in an anaerobic membrane bioreactor (AMBR). Experimental results showed that the higher the sludge concentration, the longer the ultrasonic irradiation time should be for membrane fouling control. When the AMBR operated under the conditions of crossflow velocity of 0.75m.s(-1), ultrasonic power intensity of 0.122 W.cm(-2), sludge concentrations of 4 g.L-1 and 6 g.L-1, the feasible sonication on/off times for membrane fouling control were 1 min/60 min and 3 min/ 60min, respectively. The operational experiments with the AMBR system showed that the membrane fouling was effectively controlled with ultrasonic application, and the total filtration resistance was only 30% of that without ultrasonic irradiation. Furthermore, ultrasonic irradiation in this study has slightly negative effect on the anaerobic bacteria] activity, but this did not lead to obvious decrease of COD removal rate.