Pt alloy catalysts, especially alloyed with Fe, Co, Ni, show greatly improved activities for oxygen reduction reaction (ORR) compared with Pt/C. In this work, Co (core)@PtCo (shell) is successfully synthesized based on the successful synthesis of the Co core by wet chemical way. Pt alloy shell is directly formed on Co core by the spontaneous galvanic displacement reaction in acid. The hollow Pt3Co alloy nanospheres are formed when Co@PtCo nano particles are treated in 1 mol l(-1) sulfuric acid for 8 h. It is thought that the integral and contiguous PtCo alloy shell is important to form the hollow Pt3Co alloy nanospheres. Inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), coupled with energy dispersive X-ray spectroscopy (EDX), are used to characterize crystallite morphology and composition. The ORR activities of the nanoparticles are measured with a rotating ring disk electrode (FtRDE) technique. The hollow Pt3Co alloy nanospheres with the size of about 10 nm show approximately 8 times in specific surface area activity and over 4 times in specific mass activity than that of the state-of-the art Pt/C. The electrochemical tests for duration of 5000 cycles at the sweep rate of 100 mVs(-1) between 0.6 and 1.0 V (vs. RHE) in O-2 saturated electrolyte show that the hollow Pt3Co alloy nano spheres have a remarkable stability. These render the synthesized hollow Pt3Co alloy nanospheres as a promising candidate for the ORR electrocatalyst in Proton Exchange Membrane Fuel Cells (PEMFCs). (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.