In 50%$NH_3+Air+N_2$ atmospheres, the effect of air additions on the growth characteristics of the compound layer during oxynitriding at $570^{circ}C$ for 2hr in carbon and alloy steels has been investigated. The ammount of apparent residual ammonia during oxynitriding has shown to be increased with air additions(9~36 Vol. %) and X-ray diffraction analysis of case oxynitreded has shown that the compound layer consist of ${varepsilon}-Fe_{2-3}$(N, C) phase and ${gamma}^{prime}-Fe_4$(N,C) phase. In the case of carbon steels, the thickness of oxide layer, compound layer and porous layer and the amount of ${varepsilon}-Fe_{2-3}$(N,C) phase in the compound layer were increased with additions of air in 50%$NH_3+N_2$ atmospheres. At the same gas composition, the thickenss of oxide layer, compound layer and porous layer in alloy steels showed slightly thin layer thickness compared to those of carbon steels and the ${gamma}^{prime}-Fe_4$(N,C) phase in the compound layer of alloy steels was found barely. Therefore, the most obvious effect of air addition in the gas nitriding atmosphere has been found to in crease further kinetics of nitriding reaction.