This is the first report of the application of the Taylor dispersion method to determine the dimension of the cationic surfactants protecting the nanoscopic platinum clusters. The entire size of the surfactant-protected nanoscopic platinum cluster particles, and those of surfactant themselves, determined by the Taylor dispersion, and the size of naked particles, obtained by TEM, were compared. In the case of the surfactant with a relatively short hydrophobic alkyl chain, the Stokes’ radii of clusters were shorter than those of surfactant micelles themselves, indicating that the surfactant layer is deformed by the hydrophobic interaction between the surface of metal clusters and the alkyl chain of surfactant molecules, which might be stronger than that between surfactant molecules themselves to form micelles.