The structure changes of multi-wall carbon nanotubes (MWNTs) processed by mechanical ball milling and the influence on their electrochemical hydrogen storage capacities were studied. TEM micrographs show that MWNTs are shortened and open-ended after ball milling. The effects of different MWNT type and ball milling time on the discharging capacity were investigated. Among all the samples examined, the sample of short MWNTs with diameter of 5 nm and ball milling time of 12 h has the largest discharge capacity (741.1 mAh/g). According to the analysis of Raman spectra and nitrogen adsorption experiments, it can be inferred that the micropore volume, specific surface area and appropriate defects are crucial to the storage capacity. In the cyclic voltammograms, the hydrogen desorption peak appears prior to hydrogen oxidation peak, which is attributed to the slow reaction of hydrogen oxidation at MWNTs. The results also suggest the possible existence of the strong chemisorption of hydrogen. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.