Abstract: The methods that hardenability curves were used to calculate the critical diameter of steels were introduced, including the hardenability curve slope calculation, minimum slope determination, the ideal diameter calculation by Jominy distance on the point of minimum slope, and the critical diameter calculation by the ideal diameter. The equation of the ideal diameter and Jominy distance for 50% martensite was fitted, and the equation of water-quenching and oil-quenching critical diameter and the ideal diameter was fitted. The critical diameter calculations and results of six kinds of material (40CrH steel, 45H steel, 42CrMoH steel, 20CrMnTiH steel, 20CrMnMoH steel, 40MnBH steel) were listed.