Electrocatalysts for polymer electrolyte fuel cell (PEFC) cathodes were prepared using Au-Pt core-shell nanoparticles. Polyvinylpyrrolidone (PVP)-protected core-shell nanoparticles were synthesized by simultaneous aqueous-phase reduction of Au and Pt, and they were deposited on carbon black support material. The catalyst powder was thermally processed in air to remove PVP, since the protecting polymers prevent nanoparticles from directly contacting the support material as well as reactant molecules. To avoid sintering during the thermal treatment, the effects of temperature and processing time on sintering were carefully examined. It was found that PVP was selectively oxidized and removed at 170 °C in air without notable damages to the other components of the catalyst. Stability of the core-shell catalyst in water was improved after the removal of PVP. The oxidation state of the Pt shell was found to be very close to zero. The thus-prepared Au-Pt core-shell catalyst for a PEFC cathode exhibited mass activity that was 20% higher than that of pure Pt catalyst.