Size-controlled Pt nanoparticles were prepared on multi-wall carbon nanotubes (MWCNTs) decorated with polypyrrole matrix overlayers and exhibited superior oxygen reduction reaction (ORR) performance as electrocatalysts. The copolymerization of a new Pt₄-pyrrole complex and pyrrole monomer in the presence of MWCNTs produced size-controlled Pt nanoparticles with diameters of 1.5 ± 0.5 nm. The present size-controlled Pt nanoparticles showed better durability than non-regulated Pt nanoparticles without polypyrrole and a commercial Pt/C catalyst during the ORR at the fuel cell cathode without substantial aggregation of the size-controlled Pt nanoparticles.