The factors, which affect a durability of anti-corrosive coating system for offshore structure use, are analysed and quantified. Six factors are extracted as follows ; zone of exposure, tool for painting, number of coats, surface preparation, number of coating materials, and lamination structure. Twenty kinds of materials, which compose coating systems are chosen. An effective thickness of a coation system, normalized with equivalent thickness ratios of the coating materials, is supposed. The effects of the factors and the equivalent thickness ratios on the durabilities are determined, based on the data at real environments, using the 1 st kind of quantification theory and the method of least squares. The analytical results are shown and discussed. The estimated durabilities by the analysis and those at real environments correspond fairly good. Here, the relation between the durability and the effective thickness of the coating system is assumed to be linear. It is concluded that the proposed effective thickness is nearly proportional to the serviceable period of the coating system.