© 2017 American Chemical Society. The CO oxidation reaction is an industrially important reaction; however, the practical catalysts are limited to noble metals. In this paper, we report a systematic study of the CO oxidation ability on cheap and less noble tin oxide clusters with the aim of quantitatively understanding the active sites. We synthesized size-controlled tin oxide clusters in mesoporous silica using the dendrimer templating method, employing dendritic phenylazomethine with a tetraphenylmethane core (TPMG4) as the template molecule. The clusters had different sizes depending on the added amount of SnCl2 as a precursor to TPMG4. The synthesized tin oxide clusters contained not only stable tetravalent Sn(IV) but also metastable divalent Sn(II) due to the structural stability and had a size-dependent composition. The CO oxidation activity of the tin oxide clusters increased with decreasing cluster size depending on the Sn(II)/Sn(IV) ratio. We also found the correlation between the Sn(II) fraction and the CO oxidation activity, clearly indicating that the partially reduced Sn(II) acted as the active site for the CO oxidation in the tin oxide clusters. This knowledge give a clue on how to design a highly active CO oxidation catalyst with base metal oxides.