C-shaped metal seals (C-seal) are used as static seals under the conditions such as high vacuum, high pressure, high temperature and cryogenic states.The correlation between the sealing performance and design parameters as loading force, deformation, and the surface roughness of ontacting faces have been investigated.This paper is concerned with the relationship between the surface condition of the coated-lead layer of C-seals and the surface roughness of mating flange. The effects of surface condition on leakage are examined fundamentally. Annular coated-lead layer on the surface of a C-seal prevents leakage, though there exist the fine defects or surface irregularities on the flange. It becomes clear from the test results that the surface conditions of the flange and loading force closely relate to the leakage rates with the deflection of C-seal under the. load and the behavior of the contact condition of flange surface.