The flooding, especially in gas diffusion layer (GDL), is one of the critical issues to put PEMFC to practical use. However, the experimental data of the flooding in GDL is so insufficient that the optimization design related. to the water management for GDL has not established. In this study we developed a method to estimate the water saturation, namely the ratio of liquid water to pore volume in GDL. We fabricated a simple interdigitated cell where the supply gas is enforced to flow under rib. This structure enables to estimate the liquid water ratio in GDL by the measurement of differential pressure through the cell. We operated the cell and measured the differential pressure, and succeeded in estimating the water saturation, which changed largely with changing cell operation condition. In addition to this deferential pressure measurement, we measured the ionic resistance in polymer electrolyte membrane by ac impedance method. We evaluated and discussed the influence of the water saturation on cell voltage. (C) 2007 Elsevier B.V. All rights reserved.