NO decomposition and NO reduction by CO on LaCoO₃ perovskite catalyst have been studied using the technique of thermal desorption spectrometry(TDS)under ultrahigh vacuum(UHV) condition at the adsorption temperature of 300K. About 70% of NO adsorbed molecularly on Co³⁺ ion, and the others adsorbed dissociatively on oxygen vacancy and formed various surface complexes. A part of NO adsorbed on Co³⁺ showed maximum desorption rate at 450K, and the others decomposed to form N₂ or N₂O. Pre-adsorbed CO gave little effect on NO dissociation, but in the case of pre-adsorption of NO, the post-dosed CO promoted both the dissociation of NO and the formation of N₂. The post-dosed α-CO reacted with NO adsorbed on Co³⁺ and desorbed as α-CO₂ resulting in the enhancement of NO dissociation. And the post-dosed β-CO, which adsorbed on the oxygen of the complex, also promoted the formation of N₂ by desorbing as β-CO₂.