Abstract: In order to investigate the relationship between the composition and the activity of Mn-Ce-O composites, 7 samples with Mn/Ce molar ratios of 10/0, 8/2, 6/4, 5/5, 4/6, 2/8, 0/10 were prepared by coprecipitation. NH3 was selected as a model pollutant and H2O2 was used as oxidant. The oxidation reaction in supercritical water was carried out in a continuous flow packedbed reactor at th temperature of 450℃, the pressure of 25MPa, and the residence time of 1s with the oxidant addition of 2.94 times the stoichiometric amount. The catalytic activity is greatly influenced by the catalyst composition and the catalyst with Mn/Ce mol ratio of 5/5 is found to be the best one, on which the conversion of NH3 can reach to 99.7%. The catalysts were characterized by N2 adsorption, XRD and TPR. The results indicate that there exists an interaction between Mn and Ce oxides.