The dryness of the membrane of a polymer electrolyte fuel cell (PEFC) decreases the ionic conductivity, resulting in performance reduction. In this study, the effects of external humidification to the membrane were investigated by varying the humidification side such as anode humidification, cathode humidification, and both anode and cathode humidification (called as both-side humidification). The amount of required water vapor into the gas was increased rapidly to maintain the relative humidity constant with the increase of cell temperature. The best performance of the cell was achieved by both-side humidification. However, as the humidity condition approached saturation state, anode humidification yielded comparable performance to both-side humidification. In anode humidification, the increase of the cell temperature degraded the performance, even though the amount of water supply to the membrane remained constant. At constant relative humidity conditions with anode humidification, the polarization curves of the PEFC were almost the same, regardless of the cell temperature when the relative humidity was higher than 60%.