Silica-supported bismuth molybdate catalysts were prepared by impregnation in a highly dispersed state and by coprecipitation in a largely crystallized state. Their catalytic behavior was investigated in the oxidation of propene to acrolein. The highly dispersed bismuth molybdate catalysts on silica were found to be intrinsically active but poorly selective to acrolein. When we increased the loading amount the oxidation activity drastically increased. The poor acrolein selectivity of this catalyst was improved by continuous use in the catalytic oxidation for making the particle size of the dispersed bismuth molybdate larger. The catalytic activity and selectivity were little influenced by the loading amount in the cases of the coprecipitated catalysts. The results demonstrate that, for the activity and selectivity, bismuth molybdate catalysts need to be of a certain particle size which can provide sufficient lattice oxide ions during the catalytic redox cycle.