Clay/poly(methyl metacrylate) (PMMA) organic-inorganic hybrid gas barrier layers on PP were prepared by photopolymerization. The effects of clay content on the oxygen barrier property and surface hardness of the films were investigated. Oxygen permeability coefficients of the hybrid layer was small and about eight times higher than that of poly(vinylidene chloride) (PVDC) and water vapor transmission rate of the hybrid layer was the same order of PVDC. Pencil hardnesses (50 g load) of the PP with the hybrid layers were more than B. These values were considerably higher than that of PP (6B). These properties were thought to be due to well dispersion of inorganic segments (clay) and organic segments (PMMA) in the hybrid. From the results, it was found that the organic-inorganic hybrids could be applicable to gas barrier films.