For the next generation polymer electrolyte fuel cell (PEFC), increase in operating temperature above 100 °C is desired in order to improve the cathode overvoltage. The objective of this study is to understand degradation phenomena of cathode catalysts by simulating fuel cell vehicle-s "load cycles", where Pt dissolution/reprecipitation reaction occurs. Standard membrane-electrode-assemblies (MEAs) made with Nafion® membrane were evaluated under increased temperature like 105 °C and also under reduced humidity condition like RH57%. As a result, comparing to the standard operating condition (80 °C and RH100%), Pt enlargement was suppressed at RH57% owing to the presence of smaller amount of water. However, degradation of Nafion® ionomer under low humidity condition led to lowering MEA performance. Furthermore, at higher temperature, in addition to Nafion® degradation, enhanced Pt enlargement was recognized and became a serious problem on MEA performance.