A single-cell of polymer electrolyte fuel cell (PEFC) with a Nafion membrane, a commonly used polymer electrolyte membrane (PEM), was operated at 90 degrees C and 100 degrees C, higher than the usual operation temperature. The in-plane temperature distributions on the back of separator at the anode and cathode were measured by thermograph for a range of values of flow rate and relative humidity of supply gases. The characteristics of heat and mass transfer and power generation were investigated and compared with those at the usual operation temperature. The cell voltage was found to decrease with decreasing relative humidity of supply gas irrespective of the initial operation temperature, and the cell voltage drop became larger at higher initial operation temperature of cell. These phenomena were caused by the increase in PEM resistance due to its dehydration. The cell voltage for supply gas of 40%RH decreased significantly with increasing flow rate of supply gas due to the dehydration of PEM, except at the initial operation temperature of cell of 100 degrees C. The in-plane temperature distribution on the back of the separator was more even at the anode than at the cathode due to differences in the thermal properties and flow rate of supply gases and in the water concentration distribution, except when the initial operation temperature of cell was 80 degrees C and the relative humidity of supply gas was 100%RH. The in-plane temperature distribution on the back of the separator at the cathode was more even for supply gas of 100%RH than at other values of relative humidity, irrespective of the initial operation temperature. On the other hand, the temperature increased along the gas flow from the inlet to the outlet of the cell except for supply gas of 100%RH. However, the temperature rise near the outlet of the cell at the initial operation temperature of cell of 90 degrees C was lower than that at the initial operation temperature of 80 degrees C. For supply gas of 100%RH, the in-plane temperature distribution on the back of the separator at the anode was more even at the initial operation temperature of 90 degrees C than that at 80 degrees C. It was found that controlling the wetness of PEM, i.e., optimizing the relative humidity of supply gas as well as preventing the residence of liquid water in the corner of gas channel, is important in order to obtain the high power generation performance of PEFC using a Nafion membrane at higher temperature.