A hydrothermal method has been developed to prepare size-controlled Pt nanoparticles dispersed highly on multiwalled carbon nanotubes (Pt/MWCNTs). It was found that the size of Pt nanoparticles was strongly dependent on the solution pH in synthesis. The Pt nanoparticles with mean size of 3.0, 4.2 and 9.1 nm were obtained at pHs 13, 12 and 10 separately. After Pt/MWCNTs composites were fabricated, the different properties of cyclic voltammetry and chronoamperometry in electro-oxidation of methanol were found. The results showed that the smaller diameter Pt deposited Pt/MWCNTs nanocomposites exhibited higher electrocatalytic activity for methanol oxidation. By characterization of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), size-dependent activities were identified. (c) 2008 Elsevier Ltd. All rights reserved.