Waterproof treatment by plasma polymerization was applied to the porous material of proton exchange membrane fuel cells (PEMFCs). Carbon paper used for PEMFCs was waterproofed by the gas plasma of hexafluoropropylene (HFP) prior to binding to the polymer electrolyte membrane, Nafion (R). A plasma-polymerized HFP thin film on the surface of the carbon fiber increased the contact angle of the carbon paper. The inner surface of the carbon paper was also waterproofed as well as its outer surface. It was found from the fuel cell performance test that this waterproofing method of the carbon paper was effective in the high-current density region where the condensed water product tends to accumulate. (c) 2006 Wiley Periodicals, Inc.