Abstract: Proton exchange membrane fuel cell (PEMFC) is environmentally friendly energy producer. It is one of the leading clean energy technologies being considered for transportation applications and power generation. The main hurdle limiting the large scale commercialization of PEMFC is the durability. In this paper, durability of membrane electrode assembly for fuel cell was investigated under open circuit voltage (OCV) operation at 90℃, 30% RH (relative humidity) and 50 kPa backpressure. A single cell with an active area of 25 cm² was tested. The membrane electrode assembly consists of Nafion 212 proton exchange membrane (PEM), catalyst layer and gas diffusion layer. The performance of PEMFC during the OCV operation was characterized with the help of in situ electrochemical measurements, such as polarization curves, electrochemical impedance spectroscopy (EIS) and linear sweep voltage scans (LSV). Postmortem analyses such as scanning electron microscopy (SEM), and ion chromatography were applied to identify the membrane electrode assembly degradation mechanisms during the OCV operation. The open circuit voltage degradation include reversible degradation and irreversible degradation. The open circuit voltage declined from 1.013 V to 0.794 V, and the maximum power density declined from 538.8 mV/cm² to 196 mW/cm² after 115 h of OCV operation. The on-line electrochemical measurements show that the ohmic resistance increased at the beginning and then decreased along with the OCV operation duration, while the hydrogen crossover continued to increase during the whole OCV operation. At the beginning of OCV operation, the hydrogen crossover current density was 0.867 mA/cm². However, hydrogen crossover current density became 12.64 mA/cm² after 96 h of OCV operation. Ion chromatography and SEM results showed that fluoride ions existed in both the anode and cathode effluent waterand the PEM became thinner after OCV operation. The results indicate that structural changes in PEM are the main reasons of PEMFC performance decline during OCV operation. This study indicated the PEM is very important for the durability of membrane electrode assembly.