The characteristic parameters of the gas diffusion-layer (GDL) on cell performance and mass transfer of a proton exchange membrane fuel cell have been investigated numerically. A two-dimensional, isothermal and multi-phase numerical model has been established to investigate the influence of the GDL parameters on the transport phenomenon and cell performance of PEM fuel cells. The porosity and thickness of the GDL are employed in the analysis as the parameters. In addition, the effects of liquid water and the flow direction of the fuel and air on the performance are also considered in this paper. The results show that both the porosity and thickness of the GDL affect the fuel cell performance significantly, especially the water mass transfer. It is shown that the cell performance with consideration of a liquid water effect is always less than that without consideration of the liquid water effect. In addition, the cell performance with a co-flow pattern of fuel and air is better than that with a counter flow pattern. (c) 2006 Elsevier B.V. All rights reserved.