The plasma carbunzation project was recently accepted by New Energy and Industrial Technology Development Organization（NEDO).This paper reviews the literature works on carburization process and carbide formation in steel. The text consists of 1.introduction，2.carbunzation of steel，3.heat-treatment of steel based on the phase diagram，4.heat-treatmnet after carburization，5.plasma-carburization and 6.summary，which will help to develope new carburization system and to produce high quality carburized steel.