Abstract: With the FCW of 308L super-low carbon stainless steel sheath manufactured by Tianjin Sainteagle Co. Ltd., the carbon increasing problem has been studied in this paper. After a lot of tests, it is shown that to any kind of welding method, the oxidization and deoxidization of carbon are exist simultaneously in the welding process. The increasing or decreasing of carbon in the deposit metal is depends on the carbon content of welding material. The critical value of carbon increasing for FCW is about 0.05%.