A numerical investigation of the transport phenomena and performance of a proton exchange membrane fuel cell (PEMFC) with various design parameters of the transport component is presented. A three-dimensional fuel cell model, incorporating conservations of species, momentum, as well as current transport, is used. The Bulter-Volmer equation that describes the electrochemical reaction in the catalyst layer is introduced; the activation overpotential connects the solid phase potential field to that of the electrolyte phase. Through cell performance simulation with various channel aspect ratios and gas diffusion layer (GDL) thicknesses, a slender channel is found suitable for cells operating at moderate reaction rate, and a flat channel produces more current at low cell voltage. Plots of transverse oxygen concentration and phase potential variation indicate that these oppositely affect the local current density pattern. The relative strengths of these two factors depend on the transport component position and geometry, as well as on the cell operating conditions. Consequently, the curves of cell output current density demonstrate that the optimal GDL thickness increases as the cell voltage decreases. However, at the lowest considered cell voltage of 0.14 V optimal thickness decreases as that of a thick GDL. The oxygen deficiency caused by long traveling length and clogging effect of liquid water reverses this relationship. (c) 2006 Elsevier B.V. All rights reserved.