Abstract: Mg-Co and Mg-Mn-Co composite oxides with different compositions were prepared by sol-gel method for N₂O catalytic decomposition in the presence of oxygen. Of Mg-Mn-Co catalysts, the one with higher activity was impregnated by K₂CO₃ solution to make K-modified catalyst. These catalysts were characterized by X-ray diffraction(XRD), nitrogen physisorption (BET), scanning electron microscopy(SEM), temperature-programmed reduction of hydrogen(H₂-TPR), and temperature-programmed desorption of oxygen(O₂-TPD). The effect of preparation parameters such as compositions and potassium loadings on their catalytic activity has been investigated. The results show that K-modified catalysts exhibit better activity and higher resistance towards water in contrast to un-modified catalyst due to the weakness of surface metal-oxygen bonds. Among these catalysts, 0.02K/MgMn_0.2Co_1.8O₄ is the most active, over which 97% and 60% conversions of N₂O can be reached at 400℃ after continuous running for 50 h under the atmosphere of oxygen-alone and oxygen-steam together, respectively. When the steam is switched off, the catalytic activity of 0.02K/MgMn_0.2Co_1.8O₄ can be restored to large extent, indicating the good water-resistance of K-modified catalyst.