Abstract: Properties of catalytic oxidation and gas sensitivity are very well in which CO is oxidated by La_1-xSr_xCoO₃ (x=0, 0.2, 0.4) in perovskite type catalysts. Co system perovskite catalysts have high resistance in reduction atmosphere and low resistance in oxidation atumosphere. The difference of high resistance and low resistance is very large. The catalysts are extalysts as A/F sensor materical, this study show that properties of catalytic oxidation and gas sensitivity to CO may obey the redox mechanism. The more Sr²⁺ concentration in a site of catalyst and the higher cobalt ion valence in B site of catalyst is, the more active lattice oxygen of catalyst and the better gas sensitive properties of catalyst is.