The results of reliability tests of vacuum gasket under oxidation and deoxidation are described. The heat treatment under oixdation and deoxidation is designed for cleaning of Mo first wall in JT-60. But this kind of accomplishment for vacuum flanges to be used has not been obtained yet. So we put this plan described here into practice. We used Cu gasket and stainless O-ring in ConFlat Flange under 500°C oxidation and deoxidation and obtained satisfactory results on vacuum seal (≤1.3 × 10-11Pa · m3/s). But, in Cu gasket of ICF-253, peel is observed on the gasket surface and pinlike pieces from the gasket surface and observed around the surface of the flange. These pinlike pieces may contaminate the plasma as impurities. So Ag coating on Cu gasket may be needed in the design of JT-60.