Abstract: In order to improve the reliability and wear-resistance of the sealing device in a composite transmission, based on a study of the wear mechanism and invalidation characteristics of the seal rings, a fuzzy reliability model of cast iron seal rings in composite transmission is established, applying the fuzzy theory and mathematical expression for calculating fuzzy reliability. According to the rate of wearing of the sealing ring as obtained by the wear-resistance test, the fuzzy reliabilities at different time and wearing life of the sealing ring are calculated. The result is tallied with actual situations showing that applying fuzzy mathematic to describe the variety of the wearing-resistance is suitable for the sealing ring. The method can also be applied to carry out analysis and calculation of reliability on other mechanical frictional and wearing parts.