The grain boundary formation in PtRu/C catalyst obtained in a polyol process with post-treatment was investigated by scanning transmission electron microscopy, transmission electron microscopy (TEM) and High resolution TEM. The crystalline structure and surface composition of the PtRu/C catalysts were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The electrochemical activities were evaluated by CO stripping voltammetry and linear sweep voltammetry measurements in combination with in situ IR reflection-absorption spectroscopy. As-prepared isolated spherical nanoparticles on the carbon support started to interconnect after washing procedure, and the interconnection between the particles was greatly promoted by reduction post-treatment at 80 degrees C; grain boundary formation occurred in the interconnected particles with increasing reduction temperature to 200 degrees C, and the particles reconstructed severely with further increasing reduction temperature to 400 degrees C. The defects at the grain boundary served as active sites for methanol electro-oxidation by weakening COads adsorption on Pt sites and facilitating OHads formation, and the PtRu/C catalyst treated in 5% H-2/Ar at 200 degrees C for 10 h had the greatest catalytic activity for methanol electro-oxidation among the PtRu/C catalysts treated under various atmospheres and temperatures. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.