In this research, Pt nanoparticles have been synthesized through an unsupported colloidal method to develop a compact and high-performance catalyst layer for polymer electrolyte membrane fuel cells. The intimate catalyst/electrolyte interface lowered the proton transport resistance and increased the active catalysis area in the catalyst layer, resulting in a high performance of Pt catalyst. The result also showed that the electron transport pathway should be improved to obtain a better active structure. The oxygen reduction reaction Rct resistance of the Pt colloidal catalyst layer was reduced sharply from 1.9 to 1.2 Ω, with the assistance of carbon black. Copyright © 2010 John Wiley & Sons, Ltd.