Mechanical properties of the gas nitrocarburized product depend on the surface compound layer and the diffusion zone formed. The compound layer improves the wear resistance, and the corrosion resistance. Though phase composition, pore layer and growth rate of the compound layer varies according to the treatment time, temperature and the kind of the steel substrate, they are strongly influenced by the environmental gas composition. In the current study, the growth behavior of the compound layer and diffusion zone of the carbon steel and the alloy steel upon nitrocarburizing treatment at $570^{circ}C$, and the phase composition and the variation in the growth rate of the compound layer according to the variation of the gas environment which was the medium of the nitriding and carburizing reaction were investigated.