The properties and the oxygen reduction reaction (ORR) characteristics of Pt/C and Co-rich core-Pt-rich shell/C which were prepared by the thermal decomposition and the chemical reduction methods, annealed in the various conditions were investigated. The alloying degree and grain size of Co-rich core-Pt-rich shell/C analyzed by XRD was increased from 13. 10% and 2.45 nm to 42.83% and 2.62 nm by increasing the time for annealing at 400 degrees C in N-2 (annealing condition 1) from 0 to 15 h. When the Co-rich core-Pt-rich shell/C was annealed in air at 250 degrees C and then reduced in 6% H-2 at 400 degrees C (annealing condition 2), the alloying degree and grain size were obtained to be 47.26% and 3.79 nm, respectively. The decrease in the atomic ratio of Co/Pt from 4.77 to 1.34 by annealing Co-rich core-Pt-rich shell/C in condition 1 from 0 to 15 h was deduced to be the increase in the Pt loading by the reduction of residual Pt precursor to Pt. The mass and specific activities (MA, SA) of the ORR at 0.9 V (versus RHE) on Pt/C annealed in condition 2 were obtained to be 4.11 Ag-1 and 6.12 mu A cm(-2), respectively. The MA and SA Of Co-rich core-Pt-rich shell/C annealed by condition 2 were 10.07 Ag-1 and 11.27 mu A cm(-2). (c) 2007 Elsevier B.V. All rights reserved.