A three-dimensional and two-phase numerical model is developed for a 25-cm(2) proton exchange membrane fuel cell (PEMFC) to investigate the effects of flow mode (coflow and counterflow) and relative humidity (anode 0%/100%; cathode 60%/100%) on the cell performance. Experimental studies are performed to validate this developed model. An equivalent membrane conductivity is proposed to describe the match level between current flux and membrane conductivity. It is found that the cell performance is enhanced under low relative humidity conditions because of the optimized equivalent membrane conductivity. More specifically, the voltage is improved from 0.611 to 0.637 V, and the equivalent membrane conductivity is enhanced from 10.35 to 11.11 S m(-1) by replacing the coflow mode with counterflow mode at 1000 mA cm(-2) when anode gas is dry and cathode gas is 100% hydrated. Both the anode and cathode relative humidities show an obvious influence on the PEMFC performance, and a suitable inlet humidity could ensure adequate hydration of membrane and avoid water flooding in gas diffusion layers simultaneously.