Abstract: Synthesis of low-cost, high-activity and high-stability Pt-based catalysts is of great importance to the large commercialization of proton exchange membrane fuel cell (PEMFC). Doping non-precious metals such as cobalt (Co) with Pt is attractive due to the reduced depletion of Pt and, more importantly, the enhanced activity on the oxygen reduction reaction (ORR) compared with pure Pt. In this work, carbon-supported platinum-cobalt nanoparticles (NPs) were prepared by the impregnation reduction method for the ORR catalyst. By changing the heat treatment temperature, the structure, the crystal phase and the size of the Pt₃Co nanoparticles could be controlled. TEM and XRD characterizations show that larger size NPs with higher alloying degree are obtained at higher temperature. The electrochemical results demonstrate that the Pt₃Co NPs at 800 ℃ have the highest mass activity (0.41 A/mg_Pt) and the best stability among all the samples due to their lower particle size and higher alloying degree. Further Density functional theory (DFT) calculation shows that the surface of the Pt₃Co structure with high alloying degree can reduce the rate-determining step barrier and improve the ORR activity.