In polymer electrolyte fuel cell (PEFC), it is important to understand the behavior of liquid water in gas diffusion layer (GDL) which is one of the constructional elements so as to improve the output performance and the durability. As this behavior of liquid water is attributed to not only the hydrophilicity but also inhomogeneous structure, it is needed to examine in consideration of an actual GDL structure. In this study, as the basic examination of two-phase flow analysis in an actual GDL, a simulated GDL was made by numerical analysis considering the fiber placement. Furthermore, the prediction methods for pore size distribution, permeability and tortuosity of this simulated GDL were developed with the numerical analysis. These parameters of flow and mass transfer were compared with other studies, and the validity of this simulated GDL was confirmed. In addition, effective diffusion coefficient was calculated from tortuosity in simulated GDL, and PEFC output performance was evaluated by a simple model. Moreover, the optimal GDL was examined in consideration of the effect of porosity and fiber diameter at the fiber level. (c) 2007 Elsevier B.V. All rights reserved.