It is known that mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in PEFC. In this study, the variations of the oxygen diffusion coefficient of two kinds of porous media used generally as GDL with respect to its water content (saturation) were measured by using an experiment apparatus consists of oxygen sensor based on Galvanic battery. Experimental results show that the variations of the oxygen diffusion coefficient with respect to the saturation are non-linear, and the oxygen diffusion coefficient in the porous media containing water does not only depend on the saturation.