Selective reduction of nitrogen monoxide with propane over alumina was investigated under the periodic operation of the reactant gas concentration. As expected in advance from the mechanism reported, time-averaged conversion under periodic operation exceeded that under steady operation in the range of the period tested. The averaged conversion curve showed a maximum against the period and discussion was made on the mechanism of the appearance of the maximum. It is proposed that the appearance of the periodic operation effect in the reaction system resulted from the consecutive formation of the oxidized hydrocarbon intermediate through NO2 and from the competitive consumption of the intermediate by NO and O-2, which is fundamentally different from those of NO-CO reaction and CO oxidation over noble metal catalysts where self-poisoning adsorption of CO was proposed as the reasons. Simplified model calculations were made and the proposed mechanism of the periodic operation effect in the case was estimated.