In the present study, to evaluate the conversion of intracellular PHB to methane gas, excess activated sludge with and without the accumulation of polyhydroxybutyrate (PHB) was subjected to anaerobic digestion. Excess activated sludge was collected from a laboratory activated sludge reactor and a part was fed with acetate in aerobic condition to obtain excess activated sludge with PHB accumulation. Then, excess activated sludge with and without PHB accumulation were mixed with anaerobic digested sludge, and the degradation of PHB and formation of methane gas were monitored during anaerobic fermentation at 37°C. For excess sludge with PHB accumulation, more than 75% of PHB was degraded only within the initial 2 days of incubation. Besides, 25% more methane gas was produced from excess sludge with PHB than without PHB accumulation. High production of methane gas indicates that PHB accumulated inside the cells of heterotrophic bacteria can easily be digested in anaerobic digestion process.