The selective reduction of NO (0.1%) with propylene (0.1%)in the presence of oxygen (5%) was studied using two Ag catalysts (Ag-ZSM5, Ag/Al2O3) in view of the reaction mechanism within the temperature range of 473-773 K. Ag/Al2O3 showed high and stable activity more than 24 h, while the activity of Ag-ZSM5 decreased with time on streams. The deactivation in the latter case has been attributed to the deposition of coke in the form of oligomers evoked on Bronsted acid sites of Ag-ZSM5. The ability of Ag/Al2O3 for the oxidation of NO (without the existence of reductant) was rather lower than that of Ag-ZSM5 catalyst. It was explained that the reaction is retarded on Ag/Al2O3 by the formation of nitrate species, which were produced by the reaction of NO2 with Ag2O phases. Significant differences on the ability for activation Of C3H6 over both Ag catalysts were observed. It was concluded that the reaction of nitrate species (or NO2) with partial oxidized C3H6 is essential to drive the effective reduction of NOx to N-2 and this was attributed to have high and stable activity over Ag/Al2O3 catalyst.