The dimensions of gas flow channels and walls/ribs of PEM fuel cells are optimized using a convenient mathematical model. Experimental work for several PEM fuel cells with modeling-optimized gas flow channels was conducted, and the tested results validate the modeling work and the optimization. The model considered 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, and thus 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. The modeling computation required relatively less computational time and thus can be applied to compute a large number of cases with various flow channel designs and operating parameters for optimization analysis. Optimum ratio of the width of flow channels against the walls/ribs was found from the modeling analysis. In the experimental work, PEM fuel cells were fabricated based on the flow channel dimensions optimized from the modeling analysis. Experimental results agreed with the modeling analysis satisfactorily in respect to the comparison of V-I performance between fuel cells with several optimized designs. The model is recommended as a tool for optimization design of gas flow channels for PEM fuel cells. The optimization results are of significance to the improvement of PEM fuel cell designs and performance.