The characteristics of waste activated sludge (WAS) were investigated by batch and continuous experiments using thermal pretreatment in the anaerobic digestion. The solubilization ratios of the chemical oxygen demand (COD), protein and carbohydrate increased as 14.8~21.5, 14.7∼3.5, and 21∼35 %, respectively. The maximum value of specific methane production was achieved at the thermal pretreatment of 120 ℃. The experimental and calculated data using the modified Gompertz equation for the cumulative methane production showed a good agreement with the result in the batch. In the continuous experiment the methane gas contents of thermal pretreated and controlled WAS were approximately 70 and 40 %, at steady state, respectively. For the thermal pretreated WAS, the values of k (maximum specific rate of organic carbon utilization) and Ks (half-velocity coefficient) used in the steady state equation, were 0.27 day-1 and 245.32 mg/L, respectively. These results showed that the present thermal pretreatment of WAS was useful means for the enhancement of methane production.