Accelerated membrane degradation tests were performed to investigate the membrane degradation behaviors of polymer electrolyte membrane fuel cells (PEMFCs). From the accelerated membrane degradation tests, it is elucidated that the membrane degradation behaviors greatly depend on oxygen partial pressure of the supplying gas conditions. Membrane degradation is more promoted on the anode side of the membrane under H-2-O-2 supply conditions, while the cathode side of the membrane is more promoted under H-2-air supply conditions. Oxygen crossover was quantitatively characterized by direct gas mass spectroscopy. Based on membrane thinning behaviors and oxygen crossover, we have modeled membrane thinning behaviors with a simple first-order reaction with the effect of partial pressure taken into account. The calculated values of membrane thinning are well consistent with experimental values.