Anaerobic degradation of polyhydroxyalkanoate (PHA) accumulated in activated sludge was studied to estimate its stability during sludge thickening process. Activated sludge was incubated with acetate only or acetate and propionate under aerobic condition to allow microorganisms to accumulate PHA. After the aerobic incubation, PHA-accumulated activated sludge was incubated anaerobically at 20°C, 30°C and 37°C. At 20°C for the whole incubation period of one week and at 30°C for initial couple of days, degradation of PHA was slow. These results suggested that PHA can stably be retained during sludge thickening. On the other hand, at 37°C, PHA was easily converted to volatile fatty acids (VFA) and was expected to be utilized as substrate for methane gas generation, if the thickened sludge is to be treated by anaerobic digestion. Accelerations of PHA degradation were observed during incubation, and as an explanation for it, formation of adenosine triphosphate in the fermentation of PHA to VFA was proposed. Some part of the PHA remained undegraded. As one of the keys to understand different degradation patterns of PHA, their location, whether they are inside or outside cells, was suggested.