The durability of perfluorosulfonic-acid membranes have long been the barrier for commercialization of polymer-electrolyte fuel cells. Pinholes/cracks are the typical features that impair the structural integrity and hence the mechanical durability of membranes. Although fracture mechanics approaches have been employed to address these issues, the question as to how to obtain fracture resistances of both crack initiation and propagation remains unsolved. In this paper, the fracture properties of Nafion 212 and Nafion XL membranes are investigated with R-curve method. The crack initiation point is firstly identified to calculate crack initiation resistance. For all membranes, the J-R curves increase with crack extension following power law, and the J-integrals are largest for Nafion XL in machine direction (MD) while smallest for Nafion 212 membrane. In liquid water, the fracture resistance of Nafion 212 membrane is significantly weakened while that of Nafion XL membrane is almost not influenced. Different controlling factors and mechanisms are identified, which account for the above difference. The R-curve method is also compared with the essential work of fracture method where additional information concerning crack propagation resistance can be obtained from the former one. The findings reported here provide a new insight to understand the fracture behaviors of membranes.