Gold nanoparticles supported on activated carbon (Au/C) are prepared by rapid reduction with KBH(4), after AuCl(4)(-) was partially deposited on the surface of the activated carbon by the reaction of AuC(4)(-) and ammonia. Through the characterization of the transmission electron microscope and X-ray powder diffraction, the mean diameter of the Au nanoparticles (AuNPs) decreases with the increase of the Au loading. The energy dispersive X-ray spectroscopy analysis is carried out for measuring the Au loadings of the Au/C catalysts. The results exhibit the Au/C catalyst with 20 wt% Au has the highest loading efficiency (94.5%). The origin of the catalytic activity of Au/C catalysts for the methanol electrooxidation (MEO) is investigated by the cyclic voltammetry, which indicates that the current densities normalized by the actual Au loading for the MEO increase with a decrease in the mean diameter of AuNPs by a factor of 2.42-3.17. Based on this result, the active sites (corners, edges and step sites) for the MEO are proposed. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.