In polymer electrolyte fuel cells (PEFCs), some of the water produced by the electrochemical reaction accumulates inside gas diffusion layer (GDLs), and degrades the cell performance by preventing the gas transport. In order to further improve the cell performances, it is necessary to accumulate the detailed knowledge about the mechanism of liquid water accumulation. In this study, we investigate the detailed mechanism of liquid water condensation and accumulation by performing three-dimensional unsteady numerical simulation using the volume of fluid method (VOF). The simulation, which consists of a catalyst layer (CL), GDL (substrate only), flow field was performed at 40 ℃ cell temperature and 80% relative humidity. The region where water condenses were identified by solving the temperature field and the H₂O concentration field, and it was also observed that water droplets moved between pores due to surface tension force.