Effect of additives, In2O3, SnO2, CoO, CuO and Ag, on the catalytic performance of Ga2O3-Al2O3 prepared by sol-gel method for the selective reduction of NO with propene in the presence of oxygen was studied. As for the reaction in the absence of H2O, CoO, CuO and Ag showed good additive effect. When H2O was added to the reaction gas, the activity of CoO-, CuO- and Ag-doped Ga2O3-Al2O3 was depressed considerably, while an intensifying effect of H2O was observed for In2O3 and SnO2-doped Ga2O3-Al2O3 Of several metal oxide additives, In2O3-doped Ga2O3-Al2O3 showed the highest activity for NO reduction by propene in the presence of H2O. Kinetic studies on NO reduction over In2O3-Ga2O3-Al2O3 revealed that the rate-determining step in the absence of H2O is the reaction of NO2 formed on Ga2O3-Al2O3 With C3H6-derived species, whereas that in the presence of H2O is the formation of C3H6-derived species. We presumed the reason for the promotional effect of H2O as follows: the rate for the formation of C3H6-derived species in the presence of H2O is sufficiently fast compared with that for the reaction of NO2 with C3H6-derived species in the absence of H2O. Although the retarding effect of SO2 on the activity was observed for all of the catalysts, SnO2-Ga2O3-Al2O3 showed still relatively high activity in the lower temperature region.