Among fuel cells, the polymer electrolyte fuel cell (PEFC) has the highest power density and the lowest operating temperature. PEFCs have been developed as a power source, not only for automobiles and home electronic devices, but also for portable electronics devices, such as mobile phones, laptop computers, and video cameras. Now because the solid polymer electrolyte membrane of PEFCs can conduct protons only in the presence of water, under most operating conditions, water is supplied to the solid polymer electrolyte membrane by humidifying the gas feed stream. In this study, we investigate the effect of humidification temperature on power generation performance in small PEFCs. We produced a small PEFC, with an active area of 4cm2, using Nafion as the solid polymer electrolyte membrane. By precisely controlling the temperatures of the cell, the bubblers and the piping, and the flow rates, we examined the power generation performance and cell resistance as a function of cell temperature, humidification temperature, and hydrogen and air flow rates. We clarified the power generation performance at each humidification temperature and air flow rate, and found the optimum humidification temperature for higher power performance to current density. We also estimated the level of water content in the solid polymer electrolyte membrane and electrodes. These results are useful for designers performing preliminary design studies using PEFCs as a power source for portable electronic devices.