Nafion is widely utilized as key polymer for proton electrolyte fuel cells owing to their high chemical, mechanical, and proton-conducting properties. The molecular aggregation state in Nafion films strongly affects the performance in the fuel cells. During operation process in fuel cells, peroxide derivatives such as H2O2 is generated and the chemical species deteriorates Nafion membrane, leading to a decreasing of performance of the fuel cells. The detail deterioration process including local structure, however, does not well-understand. In this work, we evaluate the local molecular aggregation state in deteriorated Nafion using microbeam wide X-ray diffraction (WAXD) measurements. Figure 1a shows WAXD line profile of before and after deterioration of Nafion membranes. The diffraction peaks were observed in ranging from 11 to 12 nm-1, which could be assigned to crystalline and amorphous structure in Nafion. The ratio of crystalline peak in deteriorated Nafion was more striking than that of initial Nafion membrane. This phenomenon might be explained following two mechanism. ① Polymer chains in amorphous domain were disconnected and were removed from the membrane. ② Polymer chains were disconnected and molecular motion of lower molecular weight is activated, leading to a formation of new crystalline domain caused by aggregation of lower molecular weight polymer chains. It is anticipated that above mechanisms occurs individually or at the same time. Though the dominant mechanism is still not clear, we can conclude that deterioration in Nafion starts from amorphous domain. To evaluate local molecular aggregation state in deteriorated Nafion, microbeams with size of 50 μm and 7 μm were utilized for WAXD measurements. Figure 1b and c show variation of ratio in crystalline domain as function of strain. The ratio of the crystalline domain in deteriorated Nafion was more striking than that of untreated Nafion. The ratio was constant when 50 μm beam was used, but not the case in 7 μm beam. This indicates that deterioration of Nafion membrane occurs 7 to 50 μm scale. The detail will be discussed in my presentation. Figure 1