|The Polymer Electrolyte Membrane (PEM) fuel cell is an ideal emerging alternative power source for transportation; however, before PEM fuel cells’ widespread use, a number of technical challenges need to be overcome, including durability which is mainly associated with three factors: mechanical, electrochemical, and thermal degradation. Among them, mechanical degradation is of paramount importance because it causes a gradual reduction of mechanical strength, toughness and, ultimately, cell performance decay. Yet, studies focusing on the mechanical degradation of MEAs and its impact on cell performance decay are relatively scarce. This thesis focuses on the early and late stages of mechanical degradation of an MEA in a PEM fuel cell. In the experimental phase, scanning electron microscope (SEM) tests detailed the initial microstructures and their changes in an MEA before and after accelerated durability testing. The possibility that large stresses, including clamping forces and hygro-thermal stresses, were the reason behind these structural changes, necessitated further studies of stress conditions in the MEA using a structure model. Techniques used to characterize the mechanical properties of gas diffusion layers (GDLs) and of catalyst layers included a microcompression tester and the nanoindentation technique. These mechanical properties guided the selection of constitutive relations in the modelling. In the modelling phase, a structure model clarified the stress and deformation of MEAs during common and cyclic operating conditions. A variety of constitutive models enabled the simulation of different materials in cells. A deformed MEA determined from the structure model enabled a more realistic method to study the cell performance under early and late stages of mechanical degradation. Results revealed that MEA’s early mechanical degradation, which is related with operating conditions, assembly methods and channel designs, had a complex effect on transport phenomenon. In addition, since an MEA’s early degradation is associated with durability, the selection of operating conditions, assembly methods and channel designs should balance both cell performance and durability. Under 2000 cyclic changes in operating conditions, the cell performance decreases about 8% merely with the mechanical degradation. Therefore, in order to increase the life span of a PEM fuel cell, it is important to find an effective approach to relief the mechanical degradation.