Atomically-precise platinum subnanocluster (0.5–2 nm) catalysts were applied to the oxidation reaction of benzylalcohol with tert-butylhydroperoxide (TBHP) as the oxidant. The subnanoclusters (SNCs) exhibited higher catalytic activities than the commercially available platinum nanoparticle catalysts. A size-dependence of the catalytic activities exhibited a volcano-shaped behavior in these oxidation reactions.