The purpose of this study is to understand the dominant factor and mechanism in coupled phenomena of heat and mass transfer and power generation in a single cell of polymer electrolyte fuel cell. Through the observation window, the in-plane temperature distribution at backside of gas channel of separator on cathode side, when it generated power, was measured by thermograph. The impact of gas supply conditions, i.e., flow rate, relative humidity of supply gas and gas channel pitch of separator on in-plane temperature distribution was investigated. The voltage to the load current, temperature, relative humidity and flow rate of supply and exhaust gas were measured. As a result, it was found the consumed gas flow rate and total voltage were almost unchanged if the gas was supplied over the stoichiometric ratio of 1.0, irrespective of relative humidity of supply gas. The range of in-plane temperature distribution was reduced with increasing excess gas supply due to the convection heat transfer by unconsumed gas flow. The power generation performance was promoted and the in-plane temperature was reduced with decreasing gas channel pitch irrespective of relative humidity of supply gas. (C) 2011 Elsevier Ltd. All rights reserved.