SCM 415 has been vacuum carburized in the carburizing pressure of 60-65kpa and the carburizing temperature of 1233k and 1273k after varied to 0-20 in the ratio of N₂/C₃H_8 and then diffusion treated for various times at 1123k. The results obtained from the experiment are as follows. 1. With increasing from 0 to 20 in ratio of N₂/C₃H_8 the sooting formation of surface after carburizing considerably decreased. 2. The hardness control and surface carbon content of carburizing surface has been modified by the addition of nitrogen to the propan. 3. The appoximate value of k is indirectry calculated at 1123k which results are obtained to 0.58×10^(-2)(wt.%.s^(-1/2)). 4. A great deal of propan by addition of nitrogen gas in carburizing gas was possible to saving without considerable change in case hardening depth. 5. The effective carburizing depth range is obtained to 0.8-1.1㎜ by diffusion temperature of 1123k after carburization at 1273k-3.6ks, and the surface hardness is increased as the increasing of T_D/T_C in our experimental condition, and the maximum hardness as reachin distance from surface is decreased.