Recently, proton exchange membrane fuel cells (PEMFCs) have received increasing attention asa clean energy conversion device due to global warming issues. These devices have outstandingadvantages including high power density, excellent energy conversion efficiency, zerogreenhouse gas emissions, and the fast transient operation. In the structure of PEMFC, the gasdiffusion layer (GDL) plays a key role. GDL which is a porous fibrous medium composed ofcarbon fiber, and a carbon-based binder has a significant effect on the fuel cell performance.it provides pathways between the catalyst layer and the flow channel and for supplying reactants,conducting electrons and heat, and removing product water [۱]. During the PEMFC performance, the GDL is compressed under the assembly pressure resulting in significant changes in itsmechanical, electrical and thermal properties. Therefore, modeling of the inhomogeneous GDLcompression under the bipolar plates (BPPs) is essential for characterizing the fuel cellperformance [۲]. For this purpose, a three-dimensional finite element model is developed in thisarticle using the ABAQUS/standard software. Due to the repetitive channel-rib pattern of the bipolar plates, only a portion of the contact area between the GDL and BPPs have been modeledand appropriate symmetry boundary conditions are prescribed on the model as shown in Figure ۱.In this figure t gdl is the GDL thickness w rib denotes the half-width of the rib, and w gdl represents the distance between the rib and the channel midplane. A nonlinear orthotropic modelhas been employed for describing the GDL constitutive behavior and a UMAT subroutine hasbeen developed to implement this model in the ABAQUS. The effect of different geometric andstructural parameters on the GDL response factors such as its porosity distribution has beeninvestigated as shown in Figure ۲ and ۳. Based on these figures, by decreasing the rib width, thesize of the low porosity area under the rib decreases correspondingly. But since the impact of therib compression less effectively transmits to regions under the channel, the GDL intrusion and the porosity increase in these regions Furthermore, decreasing the GDL thickness leads to higher porosities under the channel. It alsomakes the transition fan like region between the channel and rib becomes more localized due tothe higher concentration of rib edge effects