The hydrolysis is known to be the rate-limiting step of biological sludge anaerobic degradation. The disruptions of sludge flocs and microbial cell walls by ultrasound combined with ozone treatment (US/O-3) were investigated in laboratory-scale experiments. The results showed that temperature, O-3 dose, US energy density and pH had a positive effect on the disintegration of sludge. The organic substrates were released into the liquor, which induced the increases of soluble chemical oxygen demand (CODs) and turbidity in the aqueous phase. Accordingly, the biodegradability of sludge was improved. The CODs increased from 1821 to 2513 mg/l after reaction for 30 min when NaHCO3 was added, which indicated that the ozone molecule played a major role in the disintegration of waste activated sludge. The CODs was 2483 mg/l after 60 min O-3 treatment followed by 60 min US treatment, and it changed into 3040 mg/l after 60 min US/O-3 treatment, which proved that US/O-3 induced a synergetic effect. The pH-drop of sludge from 6.8 to 5.21 might be attributed to the increase of volatile fatty acid from 61.35 to 111.96 mg/l during the US/O-3 treatment process. (C) 2010 Elsevier B.V. All rights reserved.