Abstract: The case hardenability of the new type high-alloy steel 17CrNiMo6 was analyzed by Jominy test and delamination test aiming to find out the factors of referential carbon content.The series of hardenability curves,hardness curves along the case depth at different J distance and reference carbon content curves of the effective case were draw out.The results show that the carbon content from the surface to the effective hardened layer depth corresponding to the hardness of 550HV is not a fixed value,as is changed with alloy composition,quenching conditions and the size of the workpiece.