Polymer Electrolyte fuel cell ( PEFC ) is one of well established power-generating equipment and has become a focus of attention. The power generation process of PEFC includes a chemical reaction producing water. In a usual operating condition, water condenses inside MEA, and this liquid water accumulation is considered to affect the degradation of performance by preventing oxygen transport to the catalyst layer. In this study, to understand this phenomena, we have conducted a visualization of liquid water using X-ray microscope, and a two-dimensional numerical simulation of gas transport inside MEA. First, we have obtained liquid water distribution inside MEA by the X-ray visualization ( fig.1 ), and then incorporate the liquid water distribution into the numerical simulation as a reduced mean porosity ( i.e. reduced effective diffusion coefficient) as shown in fig.2. The resultant oxygen distribution with a condition of 3A/cm²current density, 40℃cell temperature, 80% relative humidity and 1atm gas pressure, shows lower oxygen concentration above land ( fig.3 ). In the presentation we will focus on effects of liquid water accumulation in detail by comparing with a simulation neglecting liquid water accumulation.