To maintain proper operating conditions is important to get optimal output power of a PEMFC stack. The air cooled fuel cell stack is widely used in sub kW PEMFC systems. A 500W air cooled PEMFC stack was experimentally investigated to evaluate the design performance and to get optimal operating conditions for the portable application. The relationship between the operating conditions and the performance was analyzed. The voltage distribution between cell to cell, the relative humidity of the reactant gas and the heat management by axial cooling fans were investigated as the important factors of the performance of fuel cell stack. The results can be used as design criteria for portable PEMFC under various conditions.