A new type of surfactant with a hydrophile of dendritic polyethylenimine (C-12(EI)(7)) was synthesized by a cationic polymerization of dodecylamine with aziridine and was used as a stabilizer to prepare Pt colloid, which is then used in situ to prepare carbon-supported Pt nanoparticles. The effects of supporting carbon, surfactant concentration, and calcination time on the nanoparticle size and catalytic performance were determined from the transmission electron microscopic analyses and cyclic voltammograms. In the presence of carbon, the Pt particle size increased slightly with lower C-12(EI)(7) content, while those with higher Cl-2(EI)(7) concentrations remained unchanged. For the heat-treated Pt/C catalyst, the molar ratio of C-12(EI)(7) to H2PtCl6 ([N]/[Pt] ratio) dominated the growth of Pt nanoparticles. The size decreased from 7.6 nm for a [N]/[Pt] ratio of 5 to 3.8 nm for a [N]/[Pt] ratio of 40. X-ray photoelectron spectroscopy revealed that metallic Pt-0 (81.6%) predominated the Pt species in the heat-treated catalyst, which is more than the commercial E-TEK catalyst. The data show clearly that thermal treatment had successively removed the stabilizing shells; moreover, it did not cause serious aggregation of particles in the presence of Cl-2(EI)(7) and thus enhanced the catalytic activity. The interaction between Pt and Cl-2(EI)(7) were studied and were explained in terms of a mechanism of dual-functional stabilization both on carbon and in the thermal treatment.