in　this　work,　an　isothermal,　steady-state,　three-dimensional　(3D)　multicomponent　transport　model　is　developed　for　proton　exchange　membrane　(PEM)　fuel　cell　with　straight　gas　channels.　The　model　computational　domain,　includes　anode　flow　channel,　membrane　electrode　assembly　(MEA)　and　cathode　flow　channel.　The　catalyst　layer　within　the　domain　has　physical　volume　without　simplification.　A　comprehensive　set　of　3D　continuity　equation,　momentum　equations　and　species　conservation　equations　are　formulated　to　describe　the　flow　and　species　transport　of　the　gas　mixture　in　the　coupled　gas　channels　and　the　electrodes.　The　electrochemical　reaction　rate　is　modified　by　an　agglomerate　model　to　account　for　the　effect　of　diffusion　resistance　through　catalyst　particle.　The　activation　overpotential　is　predicted　locally　in　the　catalyst　layer　by　separately　solving　electric　potential　equations　of　membrane　phase　and　solid　phase.　The　model　is　validated　by　comparison　of　the　model　prediction　with　experimental　data　of　Ticianelli　et　al.　The　results　indicate　the　detailed　distribution　characteristics　of　oxygen　concentration,　local　current　density　and　cathode　activation　overpotential　at　different　current　densities.　The　distribution　patterns　are　relatively　uniform　at　low　average　current　density　and　are　severely　non-uniform　at　higher　current　density　due　to　the　mass　transfer　limitation.　The　local　effectiveness　factor　in　the　catalyst　layer　can　be　obtained　with　this　model,　so　the　mass　transport　limitation　is　displayed　from　another　point　of　view.　(c)　2005　Elsevier　B.V.　All　rights　reserved.