This work presents variation of oxidative catalytic activities of methanol and formic acid on Pt nanoparticles of various sizes and a comparison to the results observed on Pt(111), Pt(100), and polycrystalline Pt. The Pt nanoparticles dispersed on platelet carbon nanofiber are cuboctahedral particles, whose sizes span from 5.6 to 1.1 nm. The electrochemically active surface areas, measured using charges of hydrogen adsorption/desorption and stripping of adsorbed CO, are reasonably consistent with those calculated theoretically with a simple cuboctahedron model. However, Pt nanoparticles with extremely small size (< 1.8 nm) aggregate to reduce their surface areas. The size effect of Pt nanoparticles in oxidation of methanol and formic acid is discussed in terms of specific activity (current per unit surface area) and mass activity (current per unit mass).