Fuel cells are known as one of power generation technologies with high efficiency, and are anticipated to be ecological and renewable energy. Polymer electrolyte membrane (PEM) fuel cells have excellent features including compact size and lightweight, therefore, expected to be used in automotive and portable applications. PEMs are used as an ion conductive material between electrodes in fuel cells. Perfluorosulfonic acid polymer is widely used as the electrolyte membrane because of its high proton conductivity. However, the major problem of PEMs is the mechanical degradation during use. 　In this study, toughness properties were investigated in order to clarify the mechanical fracture behavior of PEM. Flemion® CSH25 (Asahi Glass Co., Ltd) which composed of tetrafluoroethylene in main chains and sulfonic acid group in side chains was selectively used. It exhibits good proton conductivity by dissociating protons from the side chain moiety. Three types of samples with different ion exchange capacity (IEC) were used. IEC is determined by the number of sulfonic acid groups which is one of the key factors that greatly affect power generation efficiency. 　Dumbbell shaped samples were used for uniaxial tensile tests at a constant temperature and humidity. In addition, for fracture toughness evaluation, the essential work of fracture (EWF) was calculated from the load-displacement curve through double-notch uniaxial tensile test. With increasing IEC, elastic modulus and yield stress of the samples were increased. However, the lowest IEC (=1.10) sample showed the highest value of a EWF of 15.2 kJ / m2 compared with IEC 1.25 and IEC 1.43 samples. In other words, larger IEC samples showed higher mechanical properties in normal uniaxial tensile test (without double notch), but opposing to the results of EWF (with double notch). We will discuss this paradoxical result and the physical model behind in the presentation.