Abstract: With emphasis on the influence on the performance of air-breathing PEMFC (proton exchange membrane fuel cells) by various channel widths and rib widths, a coupled model was developed.In this model,natural convection in the channel and porous media was included, and concentration over potential was considered as a result of limited supply of oxygen in the catalyst layer.From the calculation, water and species distribution, temperature profile and polarization curves were simulated for the variation of cathode rib width and channel width. Results from the model indicated that concentration loss was more serious in natural convection than in forced convection, especially at a small channel opening size.The performance of air-breathing PEMFC could be improved by increasing channel opening size to some extent.Solid temperature, channel size and air flow rate interacted with each other, and the performance could not be improved infinitely by increasing channel size， and thus cathode flow field should be optimized.This work is useful for optimizing channel configuration.