Without reductive pretreatment, 10 wt% MoO3/SiO2-Al2O3 (JRC-SAH-1) exhibits high catalytic activity for propene metathesis. The Mo ions on SiO2-Al2O3 may exist as dispersed polyanions, and they can be reduced easily in contact with propene even at room temperature. The reduced Mo ions formed in the initial step of the reaction are efficient for metathesis.