This study has been performed to investigate the effect of CO_2 content on the growth characteristics of the compound layer, porous layer and corrosion characteristics of carbon steels after gaseous nitrocarburizing in 70%NH_3-CO_2-N_2 at 580℃ for 2.5 hrs. The results obtained from the experiment were the thickness of the compound and porous layers increased with increasing CO_2 contents. At the same fixed gas composition the thickness of the compound and porous layer increased with increasing carbon content of the specimens. X-ray diffraction analysis showed that compound layer was mainly consisted of ε-Fe_2-3(N,C) and γ`-Fe_4N as the increased with CO_2 contents in atmosphere, compound layer was chiefly consisted of ε-Fe_2-3(N,C) phase. With increasing CO_2 content and total flow rate in gaseous nitrocarburizing, the amount of ε-Fe_2-3(N,C) phase in the compound layer was increased. The current density of passivity decreased with increasing CO_2 content due to the development of porous layer at the out most surface of ε-Fe_2-3(N,C).