Surface-modified polymeric substrates with excellent gas barrier performances have been fabricated by coating the substrates with a silica film by means of plasma-enhanced chemical vapor deposition (PECVD) and the subsequent deposition of an organosilane layer on the silica film by a low-temperature CVD method. The hydrophobic organosilane layer on the silica-deposited substrates prevented adsorption of oxygen and water molecules on the substrate surfaces. Consequently, oxygen and water vapor transmission rates were markedly decreased down to 60% as compared to the substrates without the hydrophobic layers. In particular, the fluoroalkylsilane-coated substrate showed a water vapor transmission rate of 0.32 g/(m(2.) day), which was drastically low compared with that of the alkylsilane-coated substrate.