Electrocatalysts in ordered intermetallic phases are highly desirable for the oxygen reduction reaction (ORR) with enhanced activity and durability. Here, we develop carbon-supported ordered core-shell PtCo@Pt (O-PtCo@Pt/C with the Pt:Co atomic ratio of 1:1) intermetallic compound (IMC) nanoparticles with the metal loading as high as of 60 wt%, aiming to understand the effects of the ordered structure, metal loading and particle size on the ORR performance. The O-PtCo@Pt/C is synthesized by a two-step reduction method and followed by a heat-treatment. The mean particle diameter is about 4.8 nm with a narrow size distribution. The surface of the core-shell nanoparticles is enriched with about 3 layers of Pt atoms after the acid treatment. Noticeable lattice contraction in the O-PtCo@Pt/C has been observed by X-ray diffraction and high resolution transmission electron microscopy. Such a structurally ordered PtCo@Pt/C catalyst exhibits higher ORR activity and durability than the disordered PtCo/C and commercial Pt/C catalysts, which could be ascribed to the Pt-rich shell, ordered core structure, small particle size, lattice contraction, as well as strengthened d-hybridization. This research provides new insights to design an ordered intermetallic structure in the core and a Pt-rich surface with special facets for the excellent catalytic performance. (C) 2017 The Electrochemical Society. All rights reserved.