We successfully synthesized Au-Pt bimetallic core-shell nanoparticles by simultaneous reduction covered with a perfluorinated sulfonic acid :ionomer (PFSA) in an aqueous solution. The core-shell structure was confirmed by energy-dispersive X-ray spectrometry line analysis. Electrocatalytic activity of the Au-Pt nanoparticles for oxygen reduction reaction was higher than those of PFSA-covered. Pt nanoparticles and a conventional carbon-supported Pt catalyst.