For the application of polymer electrolyte fuel cell (PEFC) as a next generation fuel cell vehicle, an increase in the operation temperature up to 120 °C is desired in order to improve the power generation efficiency. The objective of our research is to extract problems on catalytic degradation above 100 °C with the aim of developing durable electrocatalysts for future high temperature PEFC. In this study, we have prepared standard membrane-electrode-assemblies made with Nafion® membrane and evaluated their durability against carbon oxidation in the cathode layer under increased temperature like 105 °C. As a result, comparing to the standard operation condition (80 °C and RH100%), the degradation rate of their electrochemical performance was much more increased at higher temperature. Furthermore, a collapsed carbon structure in the cathode layer was clearly observed under the high temperature operation.