A method of high-energy ball milling (HEBM) was first used to modify the structure and electrochemical performance of nickel hydroxide for the active material of Ni/MH battery. The experimental results showed that HEBM is an effective method to improve the electrochemical performance of beta-Ni(OH)(2) such as specific discharge capacity, discharge potential and cycle performance. But the performance of alpha-Ni(OH)(2) was greatly reduced by HEBM processing. The results of electrochemical impedance spectroscopy, powder X-ray diffraction and particle size distribution indicated that the improvement of the performance of beta-Ni(OH)(2) through HEBM was attributed to the reduction of the charge-transfer resistance (R-t), the diffusion impedance (Z(w)), which was resulted from the decrease of the crystallite and particle size and the increase of interlayer spacing. However, for alpha-Ni(OH)(2) samples, those structure changes after HEBM resulted in the increase of the R-1 and Z(w) and the deterioration of its electrochemical performance. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.