During the rated continuous test of a PEFC stack under stationary PEFC system conditions, the potential of several cells fell remarkably and the test could not be continue. The cause of cell potential fall was confirmed to be thinning of the polymer electrolyte membrane (PEM) in the degraded cells and cross leakage of hydrogen from the anode. Rated continuous test under different condition of relative humidity revealed that low humidity conditions accelerate cell potential fall and PEM degradation. An, open circuit voltage (OCV) accelerated durability test further revealed that PEM degradation is chemical degradation caused by three factors (low humidity, presence of hydrogen and oxygen). It is indicated that several radicals are concerned with PEM chemical degradation.