The effect of operating pressure on the water distribution and its effect on performance of proton exchange membrane fuel cell (PEMFC) was investigated. The experiment was conducted by changing the relative humidity (25%, 100%) and the back pressure value (0.2, 0.4 bar). The PEMFC was either not pressurization, or pressurization on both sides, or on the anode side or on the cathode side. In these conditions, the PEMFC showed higher performance when the cathode side was pressurization than when the anode side was pressurization. The performance difference was analyzed based on the water distribution utilizing X-ray visualization technique. Increase of water saturation in membrane was measured when the cathode side was pressurization at water deficient condition. And the saturation was getting increased as the value of pressure increased. This change caused significant increase in proton conductivity, and performance eventually. At water abundant condition, this phenomenon became weaken and performance difference became narrow. However, increase in pressure on the cathode side increased the air penetration through water and reduced the concentration over-potential. Precedent researches concentrated on the reaction kinetics to analyze the effect of pressure, but the change of water distribution should be considered simultaneously to understand the effect of pressure. (C) 2017 Elsevier Ltd. All rights reserved.