Abstract: The distribution of B and N in 18CrMnB steel during the hardening at 870℃ is investigated by means of the microphotography, and CB is used as a parameter to show the distribution of N in the steel. The results obtained are as follows: the hardenability increases with parameter C_β, and if its value is between 14.5 though 27.5, the high hardenability of 18CrMnB steel is obtained. C_β is a useful parameter for controlling the hardenability of this steel.