Numerical simulation of transport phenomena in a six-cell PEM fuel cell stack is performed to adjust the gas channel size of individual cells to obtain evenly distributed cell voltages. Commercial software, CFD-ACE+, is used as the solution code, and the effects of various combinations of geometric parameters of the channels are investigated, including the channel width ratios (Lambda) and the heights of the gas channels (h). Furthermore, a six-cell fuel cell stack was assembled for experiments, and the polarization curves of the stack were measured. With the help of the experimental data, the values of the physical and electrochemical parameters adopted in the computation model are determined, and the computation model is then used in numerical simulation. Under the assumption of no dry out in the cells, an even voltage distribution can be obtained if the distribution of reactant gas is uniform among the cells. In this study, better combinations of the channel width ratios and the heights of gas channels for individual cells are proposed to yield a uniform distribution of the reactant gases in the cells. An adjustment of the geometric parameters of gas channels that leads to evenly distributed voltages is attempted, and a 16.5% increase in the performance of the fuel cell stack is observed simply by adjusting the sizes of the gas channels. (C) 2009 Elsevier B.V. All rights reserved.