Abstract: The paper describes the experiment of 304 ss foil exposed to high temperaturesodium at working temperature of fuel cladding materials for LMFBR. Microstuc-tures are observed, phase distribution is measured and composition analysis beforeand after exposure to high temperature sodium is performed. According to carbonequilibrium equation reported in literature by abroad, the carbon activity and theequilibrium concentration of carbon in sodium at 650℃ are calculated. The effectof both heat treatment (temperature, time) and original carbon content onmeasurement of carbon activity are discussed and the experimential correlation isrevised.