Abstract: On the basis of three structure criteria of hardenability (D_I90%M,D_I99.9%M and D_I50%M) of 20CrMnTi, 20MnVB and 20CrNiMo steel, according to the case hardenability structure criterion (90%M), established regressive equations which contained the term of interaction effect between alloy elements on hardenability, and could continuously calculated the values of case hardenability(JD_90%M——on the top-quenched specimen,the distance from the top to the 90% martensitezone). The calculated values by these equations have shown satisfactory agreement in comparision with the experimental data of aforementioned three steels.