The performance of an ah-breathing polymer electrolyte membrane fuel cell (PEMFC) stack was evaluated under different environmental conditions. The humidity of the surrounding air significantly affected the performance of the stack. When the humidity was less than 10% RH at 35 degrees C, the stack lost almost 95% power. Acceptable operating temperature for the stack would be from 20 to 40 degrees C for > 36% RN. An empirical equation was proposed to analyze the performance of the PEMFC stack instead of only for a single cell. The results calculated with the empirical equation are: excellently fitted to the experimental polarization curves at various humidity and temperatures. The impedance and Tafel slope of the stack both increase with decreasing either the environmental humidity or temperature.