The effect of changing operation temperature and H and O-2 concentration in supply gas on the i-V characteristic of a small PEFC was examined experimentally. Cell performance was found to decrease with decrease in H-2 and O-2 concentrations and the influence Of O-2 concentration was larger than that of H-2. When the relative humidity of the supply gas at both anode and cathode was 100%, no influence of operation temperature could be found. Furthermore, a model was created that can express these influences on PEFC reaction characteristics. Combination of these model equations with gas flow analysis allows the reaction characteristics to be examined in an actual scale PEFC.