The effect of thermal pretreatment on the characteristics of degradation of waste activated slduge (WAS) in anaerobic digestion was investigated by laboratory-scale batch and continuous experiments at 35±1°C. The pretreatment temperatures of WAS ranged from 62 to 175°C and the pretreatment times were 15, 30, 60, 90 and 120 minutes. The organic particulates in WAS were solubilized to soluble carbohydrates, lipids and proteins or converted into the lower molecular weight compounds, such as volatile fatty acids, by the thermal pretreatment. Through the thermal pretreatment. the anaerobic degradability of WAS and the gas production from WAS were greatly increased, and the retention time necessary for anaerobic digestion was possible to be reduced by 5 days. The optimum pretreatment temperature and pretreatment time for the upgrading of anaerobic digestion of WAS were 170°C and 60 minutes, respectively. At this pretreatment condition, the COD removal and the gas production were markedly increased. At the digester with retention time from 5 to 10 days, the COD removal efficiency reached over 60% and gas production ranged from 223 to 235ml·gCOD−1 which were about 2 times those of the control. The numbers of methanogenic bacteria existing at retention times from 1.5 to 10.0 days ranged from 107 to 108 MPN·ml−1, and reached the maximum population level at the retention time of 5 days. The increase in solubilization and anaerobic degradation of major organic compounds in WAS by thermal pretreatment decreased in the following order: carbohydrates > proteins > lipids.