Silica-supported poly(maleic acid-co-styrene)-triruthenium clusters have been prepared by carboxyl exchange of (C2H5COO)6Ru3(mu3-O)(H2O)3+C2H5COO- with carboxyl groups of silica-supported poly(maleic acid-co-styrene). The supported cluster displays a high activity in the oxidation of styrene under very mild conditions. The main product of the oxidation is benzaldehyde which results from the oxidative cleavage of styrene by molecular oxygen. During the catalytic reaction, the cluster on the support degrades most likely to monoruthenium species such as (circled P-COO)3Ru(V)=O and (circled P-COO)3Ru(III), and the pentavalent ruthenium species may be the catalytically active species in the oxidation. Therefore, the supported cluster could be recycled more than eight times without a noticeable decrease of its activity although it is already fragmented in the first cycle. A possible mechanism for the styrene cleavage is proposed.