The effects of polycrystalline nickel and nanocrystalline nickel on the electrochemical properties of the MmNi(3.8)Co(0.75)Mn(0.4)Al(0.2) hydrogen storage alloy with an emphasis on the influence of nanocrystalline nickel prepared by mechanical milling are investigated. The electrochemical properties such as the activation, plateau potential and discharge capacity are improved after the modification with the nanocrystalline nickel. The result of linear polarization measurement shows that the exchange current density increases and the polarization resistance decreases after the modification. The result of Tafel polarization measurement also demonstrates the same consequence, shows an increase of the limiting current density and an excellent ability of antioxidation or anticorrosion, and indicates an improvement of the kinetics of the hydrogen absorption/desorption after the modification. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.