This work numerically investigates the influence of the channel cross-section aspect ratio (defined as the ratio height/width) on the performance of a PEM fuel cell with serpentine flow field (SFF) design. The local current densities, velocity distributions, liquid water concentration in the membrane, hydrogen and oxygen concentrations and temperature were analyzed in the PEM fuel cell for 10 different aspect ratios, varying between 0.07 and 15, to understand the channel cross-section aspect ratio effect. Tie area of the channel cross section (1.06 mm(2)) and the total effective reactive area of the PEM fuel cell (256 mm(2)) were maintained constant in all cases. The obtained results show that at low operating voltages the cell performance is independent of the channel cross-section aspect ratio. At high operating voltages, where the influence of mass transporting velocity is predominant, as the channel cross-section aspect ratio increases the cell performance is improved. The models with high aspect ratio show, in general, more uniform current distributions, with the higher maximum and minimum intensity values, temperature distributions with smaller gradients and a superior contain of water in the membrane, which allows to obtain a higher performance. From these models the 10/06 and 12/05 aspect ratio present the best combination of variables, as shown by their polarization curves. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.