The γ-Fe₂O₃, compounds were oxidized in the furance after gas detecting sensor made molding as the Fe₃O₄, the synthesized Fe₃O₄, by ferrous sulfate and sodium hydroxide. Their sensities on carbon monoxide, ethyl alcohol and L. P. G. were measured at various temperatures, respectively. And then their electrical resistivities, thermal properties (D.T.A. & T.G.A.), were examined about their having an effet on the gas-sensing in company with the effect of pure γ-Fe₂O₃and the detecting sensor γ-Fe₂O₃which reacts with K₂CO₃in the hydrothermal coundition, and the electrical conductive mechanism was reflected simultaneously. It was observed that the electrical conductivities and response ratios showed highest value at the endothermic temperature part, 300∼350℃ of D.T.A.-curve. Consequently, the response ratios and response times of the hydrothermal detecting sensor were higher than that of pure detecting sensor, the specific surface areas were the highest at the endothermic range of D.T.A.-curve. These response ratios of detecting sensor for K₂CO₃, process at hydrothermal condition on carbon monoxide appeared higher than that on ethyl alcohol and in case of L.P.G. last.