The mechanism of selective catalytic reduction of NO by C2H4 over Cu-MFI zeolite has been studied by a pulse technique, in which the reactivity of ethene, NOX, nitrogen- or oxygen-containing compounds has been determined. It follows that the major route to form N-2 is the reaction of the mixture of NO and O-2 with surface adsorbates of C2H4. Ethene adsorbed on Cu-MFI is classified into two kinds of hydrocarbon species. One is active for the reaction with NOX and another is inert. Oxygen molecules are effective to increase the amount of the active hydrocarbon species for NOX. NO as well as NO2 would directly be included in the selective reduction as an active NOX species.