The selective reduction of NOx by reductants such as C3H6 and CO in oxidizing feedstreams simulated exhaust from automobile engine and three-way behavior around the stoichiometric point have been investigated on trimetallic PtMoNa/SiO2 catalysts, over a wide range of temperatures compared with bimetallic PtMo/SiO2 catalyst and monometallic Pt/SiO2 catalyst. The simultaneous addition of Mo and Na to Pt catalysts has been found to improve the following reaction characteristics on selective reduction of NOx and three-way activity of conventional Pt catalyst. The temperature window on selective reduction of NOx on trimetallic PtMoNa/SiO2 catalysts was found to be wider and to shift at higher temperature than that on bimetallic PtMo/SiO2 and monometallic Pt/SiO2 catalysts under lean static conditions. The redox ratio window, in which three-way activity occurred on trimetallic PtMoNa/SiO2 catalysts, was also found to be wider than that on bimetallic PtMo/SiO2 catalysts and monometallic Pt/SiO2 around stoichiometric point. On the other hand, XPS, IR and CO adsorption data indicated that the oxidation of Pt on PtMoNa/SiO2 catalysts was depressed by the added Mo and Na even under excess oxygen conditions, so that the reaction characteristics of trimetallic PtMoNa/SiO2 catalysts was improved.