The effect of Air/C₃H_8 gas addition on the compound layer growth of steels nitrocarburised in NH₃+Air+C₃H_8 mixed gas atmospheres was investigated. It is considered that amount of residual NH₃was varied according to alternation of Air/C₃H_8 mixing ratio and volume content. The compound layer formed from nitrocarburising was composed of ε-Fe_(2-3)(C, N) and γ`-Fe₄(C, N). According as Air/C₃H_8 mixing ratio increased, the superficial content of γ`-Fe₄(C, N) within the compound layer was increased, at the same time the growth rate of compound layer and porous layer was increased. In the case of alloy steel at the fixed gas composition, the growth rate of compound layer and porous layer was worse than carbon steel and compound layer phase composition structure primarily consisted of E phase. As the carbon content of materials was increasing in the given gas atmospheres, the growth rate of compound layer and porous layer was increased and the superficial content of ε-Fe_(2-3)(C, N) within the compound layer was increased.