Mechanical reliabilities of membrane electrode assemblies (MEA) in polymer electrolyte fuel cells (PEFCs) are a major concern to fuel cell vehicles. Especially, MEAs are designed to be thinner for obtaining higher generating performance and reducing cost. Proton exchange membranes (PEM) in MEA are especially important parts. When PEFCs generate power, MEAs are in high temperature and water is generated. Hygrothermal cyclic conditions induce the mechanical stress in MEA and cracks are formed on catalyst layers. Once cracks form on catalyst layers, cracks may propagate into PEM or on the interface between the catalyst layer and PEM. The failures of PEM induce the leak of fuel gases and result in the shortage of output power. Therefore, in order to ensure the durability of thin MEAs, it is important to know the fracture resistance of PEM. The deformation of PEM is constrained by coated catalyst layers and crack propagates into thickness direction of PEM under the constrained condition. However, there are no available data for the fracture resistance of cracks propagating into thickness direction in MEA, because of the difficulty of measurements. Therefore, in this paper, we try to measure the fracture resistance of cracks propagating into thickness direction in PEM under several environmental conditions. Elastic-plastic fracture toughness tests for PEM were carried out in temperature and humidity controlled chamber. The results showed that the fracture resistances of crack propagation into thickness direction of PEM were strongly affected by temperature and humidity conditions.