Ultrasound treatment of primary and secondary sludges was conducted to improve the qualities of sludges for the anaerobic digestion. The impacts of different sonication times, sonication densities and solids concentrations on ultrasonication efficiency were examined. The experimental results indicated that the significant reduction in particle size and increase in soluble organics could be achieved, implying that ultrasonication could offer a feasible treatment method to efficiently disintegrate sludge. The greater decrease in particle size and increase in soluble organics of sludge indicated that the secondary sludge has a more remarkable improvement after sonication over the primary sludge. With respects to the extent of disintegration and energy consumption, higher sonication density performed more effectively in terms of specific energy. There exists an optimal solids concentration range for both the sludges for optimum sonication. Within the optimal solids concentration range, efficient sonication can be effected and sludge would be disintegrated efficiently. The ultrasound would be attenuated by scattering and absorption if the solids concentration exceeds the optimal range. It appeared from the study that the mechanical shear forces caused by ultrasonic cavitation could be a key factor for sludge disintegration and collapse of cavitation bubbles could significantly alter the sludge characteristics.