The effect of coexisting SO2 on the catalytic activity of Ga2O3-Al2O3 prepared by impregnation, coprecipitation and sol-gel method for NO reduction by propene in the presence of oxygen was studied. Although the activity of Al2O3 and Ga2O3-Al2O3 prepared by impregnation (Ga2O3/Al2O3(I)) and coprecipitation (Ga2O3-Al2O3(CP)) was depressed considerably by the presence of SO2, NO conversion on Ga2O3-Al2O3 prepared by sol-gel method (Ga2O3-Al2O3(S)) was not decreased but increased slightly by SOz at temperatures below 723 K. From catalyst characterization, SO2 treatment was found to cause two important effects on the surface properties: one is the creation of Bronsted acid sites on which propene activation is promoted (positive effect), and the other is the poisoning of NOx adsorption sites on which NO reduction proceeds (negative effect). It was presumed that the influence of SO2 treatment on the catalytic activity is strongly related to the balance between the negative and positive. The activity enhancement of Ga2O3-Al2O3(S) by SO2 was accounted for by the following consideration: (1) increase of the propene activation ability by SO2, (2) incomplete inhibition of NOx adsorption sites by SO2.