In order to predict effects of additional gas concentration on carburization of metallic surface, numerical analysis of chemical kinetics and ab initio calculation were conducted in the present study. Time history of mole fraction of chemical species in reaction gas were calculated by CHEMKIN II and GRI-Mech3.0 with changing additional gas conditions. Reactivity of CO_3 CH_4,C_2H_2 C_2H_4 and C_2H_6 in reaction gas was estimated qualitatively by GAUSSIAN98 in a simple model system of two Fe atoms. Effects of gas addition on carburization were estimated simply by putting calculation results together.