Carburization and decarburization behavior in sodium have been examined for Fe-9Cr-Mo ferritic steels exposure to flowing sodium with containing 0.0049, 0. 016 and 0. 16 wppm carbon at temperatures between 723 and 873 K. The Fe-9Cr-Mo ferritic steels were more resistant to carbon transfer than 2,Xi'Cr-1Mo steel. The carbon transfer and its gradient in Fe-9Cr-Mo ferritic steels was closely related with carbon concentration in sodium, and carburization and decarburization behavior depended on the sodium conditions. In the case of the application of Mod. 9Cr-1Mo steel to heat transfer tubes of steam generator of LMFBR, carburization behavior was observed on the conditions that sodium inlet temperature was 778 K and carbon concentration in sodium was 0. 01-0.10 wppm. The mean carbon contents of heat transfer tubes after the operation of the steam generator for 200,000 h were estimated to be about 0. 103-0. 148 wt% comparing with initial carbon contents of 0. 10 wt%. Consequently it is expected that high-temperature strength of Mod. 9Cr-1Mo steel would retain sufficient strength and ductility for such operating conditions.