Two series of the Ce-Mo oxide catalysts for the partial oxidation of toluene to benzaldehyde have been prepared, respectively, by the sol-gel and coprecipitation methods and characterized by using various techniques. The results have shown that the catalytic reactivity is strongly composition related; and ultrafine Ce-Mo oxide particles prepared by the sol-gel process are apparently a better catalyst than large Ce-Mo oxide particles with a same composition but prepared by the coprecipitation method. The highest yield of benzaldehyde is obtained when the composition of the ultrafine particles reaches the vicinity of Ce/(Ce + Mo) = 0.50, corresponding to a catalyst consists of both CeO2 and Ce-2(MoO4)(3). The reaction mechanism and the promotion effect of the excess CeO2 in the ultrafine Ce-Mo oxide are discussed to explain why ultrafine Ce-Mo oxide particles prepared by the sol-gel method are good for the partial oxidation of toluene.