In this paper a fractal permeability model for the gas diffusion layer (GDL) of PEM fuel cells (PEMFCs) is presented. The model accounts for the actual microstructures of the GDL in terms of two fractal dimensions, one relating the size of the capillary flow pathways to their population and the other describing the tortuosity of the capillary pathways. In addition, the gas molecule effect is considered by using the Adzumi equation. The fractal permeability model is found to be a function of the tortuosity fractal dimension, pore area fractal dimension, sizes of pore and the effective porosity of porous medium without any empirical constants. mercury-intrusion porosimetry was used to measure the microstructures of the GDL. Based on scanning electron microscope (SEM) images, two fractal dimensions are determined by the box-counting method. To verify the validity of the model, the predicted permeability data of the present fractal model were compared with the experimental data supplied by Toray Inc. It is found that the permeability prediction of the model was in accordance with experimental data. This verifies the validity of the present fractal permeability model for the GDL.