PtSn bimetallic catalyst nanoparticles were grown on highly oriented pyrolytic graphite (HOPG) surfaces using the sequential deposition and co-deposition of Pt and Sn. The surface morphology and composition of the prepared PtSn catalysts were investigated by scanning tunneling microscopy and CO adsorption experiment. It is found that the structure of the PtSn catalysts can be controlled by the deposition process. The deposition sequence of 1st Pt and 2nd Sn and co-deposition of Pt and Sn produce Sn-shell and Pt-core structure, while the deposition sequence of 1st Sn and 2nd Pt leads to the growth of individual Pt and Sn particles. The formation of various PtSn bimetallic structures was attributed to the different surface energy of Pt and Sn, and the interaction of the metals with the HOPG surfaces. (C) 2007 Elsevier B.V. All rights reserved.