The structure, morphology and electrocatalytic characteristics of nickel powders treated by mechanical milling have been studied. The milling process is controlled by the processes of cold welding and fracture. During the cold welding dominant stage, severe deformation of the particles occurs. The nickel particles are flattened and cold welded together, leading to the formation of a lamellar structure and an increase of the average particle size. During the fracture dominant stage, the average particle size decreases and quasi-spherical particles form. Nanocrystalline structure is obtained when the milling time reaches 5 h. The energy produced by mechanical milling is mostly introduced into refining the grain size during the milling period of 5 h and expanding the unit cell during the milling period from 5 to 40 h. The formation of nanocrystalline structure with a proper density of dislocations by mechanical milling can substantially improve the electrocatalytic activity of the nickel powders for hydrogen adsorption. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.