A two-dimensional, steady state model for proton exchange membrane fuel cell (PEMFC) is presented. The model is used to describe the effect of flow mode (coflow and counterflow), operation conditions and membrane thickness on the water transport, ohmic resistance and water distribution in the membrane, current density distribution along the channel and performance of PEMFC. Effect of liquid water on the transport in the two-phase region of cathode diffusion layer was considered. Water transport in the membrane by electro-osmosis drag, diffusion and convection were combined in this model. The model predicts that the dry reactant gases can be well internally humidified and maintain high performance when PEMFC is operated in the counterflow mode without external humidification. Counterflow mode does not show any advantageous while the reactant gases are high humidified or saturated. Compared to the coflow mode, counterflow mode improves the current density distribution with dry or low humidity gases. The higher the anode is humidified, the more water will migrate from anode to cathode. The modeling results compare very well with experimental results. (C) 2003 Elsevier B.V All rights reserved.