Polymer electrolyte membrane fuel cell (PEMFC) is one of the promising electricity generating technologies with a wide range of applicability; however, it needs further improvements to be commercially viable. The design of a PEMFC plays a key role in its viability, and is often reduced to the design of gas flow channel (GFC) at the cathode side. In this study, it is attempted to figure out the optimal dimensions (i.e., width and height) of the rectangular cross sectional area of the cathode GFC of a PEMFC via numerical examination of various sets of dimensions. The optimization procedure is carried out for two different objective functions (the maximization of the maximum power and the maximization of the average power over a range of operating voltages) as well as for different sets of operating conditions (cell temperature, operating pressure, and stoichiometry and relative humidity of inlet gases). To the best of authors' knowledge, the following observations may be considered to be the contributions of the present work to the subject: First, the influence of cross sectional dimensions on the PEMFC performance is considerable, and this considerable influence is not limited to a specific set of operating conditions. Second, the performance of the PEMFC may both deteriorate and improve with the channel width or height, depending on its operating conditions as well as on its current dimensions. Third, there exists no single optimal cross section for different sets of operating conditions. Fourth, the polarization curves of two different cross sections may intersect, and as a result, one cross section may have a greater maximum power but at the same time lower average power in comparison to the other one. And fifth, among all the operating parameters, the relative humidity of inlet gases has the greatest effect on the optimal cross sectional dimensions. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.