To enhance the efficiency of anaerobic digestion, we examined the effects of ultrasonic pretreatment on the solubilization of waste activated sludge (WAS) and on methane generation. Organic substances in WAS were solubilized by ultrasonic pretreatment. In the digestion with WAS pretreated by ultrasonic pretreatment for 30 min, the total quantity of generated methane increased by 64%, as compared with experimental control (non-treatment). In addition, the relationships between methane generation and concentrations of soluble protein, carbohydrate and volatile fatty acid (VFA) were also examined.