A coupled 3D mathematical model for the real geometry of an air-breathing proton exchange membrane (PEMFC) was developed and validated by experimental data. The free convection in the cathode side was included in the model. The concentration over-potential was considered as a function of mass transfer coefficient of oxygen in the catalyst layer. Governing equations possess the features that fluid dynamics, mass/heat transfer are coupled with the electrochemical reactions. The model was solved in a commercial software STAR-CD based on the finite-difference and finite-volume methods, and the electrochemical features and water transport in membrane are solved simultaneously through a user-specific subroutine. To investigate the effect of channel configuration on air-breathing fuel cell performance, calculations for three different widths of channels have been executed. Results show the best performance can be obtained in the cell with cathode channel width of 3 turn (open ratio of 75.9%). (c) 2005 Elsevier B.V. All rights reserved.