A mathematical model is developed for evaluating the effects of various channel dimensions on the performance of an air-breathing polymer electrolytes membrane fuel cell (PEMFC). The model, which is based on Nguyen's model, has been extended to include the natural convection to consider buoyancy effect in the channels, electro-chemical reaction in the catalyst layer, and concentration overpotential due to mass transportation limitation. Results from the model indicate that the concentration loss is more serious in natural convection than in forced convection, especially at small channel width, and the performance of air-breathing PEMFC could be improved by increasing the channel width to some extend. Results also show that the temperature, channel size, and air flow rate influence each other, and the performance cannot be improved infinitely by increasing the channel size, and thus the cathode flow field should be optimized. This model provides insights into many design issues of air-breathing fuel cell, and can be easily used as an optimal design tool for air-breathing PEMFC. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.