A three-dimensional, two-phase, non-isothermal model has been developed to explore the interaction between heat and water transport in proton exchange membrane fuel cells (PEMFCs). Water condensate produced from the electrochemical reaction may accumulate in the open pores of the gas diffusion layer (GDL) and retard the oxygen transport to the catalyst sites. This study predicts the enhancement of the water transport for linear porosity gradient in the cathode GDL of a PEMFC. An optimal porosity distribution was found based on a parametric study. Results show that a optimal linear porosity gradient with epsilon(1) = 0.7 and epsilon(2) = 0.3 for the parallel and z-serpentine channel design leads to a maximum increase in the limiting current density from 10,696 Am-2 to 13,136 Am-2 and 14,053 Am-2 to 16,616 Am-2 at 0.49 V. respectively. On the other hand, the oxygen usage also increases from 36% to 46% for the parallel channel design and from 55% to 67% for the z-serpentine channel design. The formation of a porosity gradient in the GDL enhances the capillary diffusivity, increases the electrical conductivity, and hence, benefits the oxygen transport throughout the GDL The present study provides a theoretical support for existing reports that a GDL with a gradient porosity improves cell performance. (C) 2010 Elsevier Ltd. All rights reserved.