A novel stepped flow field is proposed to improve the performance of a proton exchange membrane fuel cell. A three-dimensional model that accounts for activation polarization, ohmic polarization and concentration polarization is developed to achieve this. Numerical results demonstrate that overpotential values decrease in the order eta(act.c) > eta(por)(ohm) > eta(el)(ohm) > eta(act.c) > eta(conc.c) > eta(conc.a) at high current density. Generally, the concentration polarization can beignored becauseofits lowvalue. Usingastepped flowfield improves the reactant concentration distribution, local current density distribution, water vapor concentration distribution and cell performance. If the number of steps approaches infinity. the stepped flow field extends to a tapered field that has the lowest cell performance. The lower the number of steps and the height of the outlet channel, the greater is the improvement in reactant diffusion to the porous layers, local current density distribution. water management and cell performance. (c) 2008 Elsevier B.V. All rights reserved.