Anaerobic digestion experiments at 35°C of primary and surplus sludge were carried out, showing that primary sludge was superior in digestability and gas production rate to surplus one. The results suggsted to effective to promote digestion process. Material-and energy-balances show the advantageof thermal pretreatment plus digestion process over directdiges tion method. The studies on the effects of mixing and temperature shock in digester show that agitaton is one of the most important factors controlling digestion process, while effect of temperature shock is small in mesophilic digestion.