Membrane temperature field of a polymer electrolyte fuel cell (PEFC) has been visualized expert. mentally. PEFCs need further breakthrough for deployment in the market. One of the major issues is the temperature management of the polymer membrane and the, whole cell that strongly govern system performance through electrochemical reactions, ion transport, water management, and gas supply. The temperature field of the membrane, however had not been visualized due to the cell configuration. In our experiment, the thermography technique is applied to visualize an operating test cell. Despite the unique configuration, measured i-V characteristics guarantee the cell performance. The visualization results revealed several important characteristics that help us understanding the physics and suggest design knowledge. One major result is the existence of so called a hot spot. The membrane does have a temperature distribution, and a local temperature maximum may exceed the membrane design limitation. This trend, of course, is not favorable for design purposes. Also, the impact of the major operation parameters, such as current density, humidification, and gas flow configuration, have been clearly exhibited. The experimental results are examined by using the results of our previously developed numerical code. The code includes the conjugate nature of the electrochemical reaction and the heat and mass transport processes. By comparing the experiment and the calculation, the mechanisms of the hot-spot generation and the parameter dependence have been explained. The results revealed. the physics and suggested essential design criteria.