The objective of this work is to optimize the dimensions of gas flow channels and walls/ribs in a proton-exchange membrane (PEM) fuel cell. To achieve this goal conveniently, a relatively easy-to-approach mathematical model for PEM fuel cells has been developed. The model was used for the design optimization of fuel cells, which were fabricated and experimentally tested to compare the performance and examine these optimization effects. The model analyzes the average mass transfer and species' concentrations in flow channels, which allows the determination of an average concentration polarization, the humidity in anode and cathode gas channels, the proton conductivity of membranes, as well as the activation polarization. An electrical circuit for the current and ion conduction is applied to analyze the ohmic losses from anode current collector to cathode current collector. This model needs relatively less amount of computational time to find the V I curve of the fuel cell, and thus it can be applied to compute a large amount of cases with different flow channel dimensions and operating parameters for optimization. Experimental tests of several PEM fuel cells agreed with the modeling results satisfactorily. Both simulation and experimental results showed that relatively small widths of flow channels and ribs, together with a small ratio of the rib's width versus channel's width, are preferred for obtaining high power densities. To further demonstrate the advantage of optimized fuel cell designs, two four-cell stacks, one with optimized channel/rib designs and the other without, were compared experimentally and a much better performance of the one with the optimized design was confirmed. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.