Fullerene-like orthorhombic-structured Co3C nanoparticles have been synthesized by direct ball-milling of Co and graphene (GE) powders with different Co/GE weight ratios. Electrochemical measurements showed that the Co3C nanoparticles displayed excellent electrochemical hydrogen storage capacities and the maximum capacity reached 1415 mA h/g (5.176 wt% hydrogen) at room temperature and ambient pressure. The reaction mechanism and the reasons for the differences of the Co3C electrodes were also investigated. It was found that the quasi-reversible Co3CHx/Co3C reaction was dominant for all the Co3C electrodes. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.