Reactive ball milling was employed to synthesize nanocrystalline MgH2 powders using a high-energy ball milling of pure Mg powders under 50 bar of a hydrogen gas atmosphere. The end-product of MgH2 powders obtained after 200 h of a continuous reactive ball milling time composed of fine grains (similar to 7 nm in diameter) of gamma and beta phases. A new catalytic agent of big-cube Zr2Ni nanocrystalline phase, which is proposed in the present study for improving the hydrogenation/dehydrogenation kinetics of MgH2 powders, was obtained upon high-energy ball milling of tetragonal-Zr2Ni powders for 150 h. The as-ball milled Zr2Ni powders consisted of ultrafine grains with an average grain size of 6 nm in diameter. The as-prepared MgH2 powders were mechanically doped with 10 wt% of big-cube Zr2Ni powders for 50 h, using high-energy ball mill under a hydrogen gas atmosphere for 50 h. The powders obtained after 50 h of milling enjoyed homogeneous morphology and uniform composition close to the starting nominal composition. Moreover, this binary nanocomposite system possessed superior hydrogenation/ dehydrogenation kinetics at 250 degrees C, as suggested by the short time required to absorb and desorb 5.1 wt% H-2 within 100 s and 613 s, respectively. At this temperature, the synthesized nanocomposite powders possessed excellent absorption/desorption cyclability of 2546 complete cycles within 1250 h. However, a minor degradation (similar to 0.5 wt% H-2) in the hydrogen storage capacity was observed between 300 h and 2546 h of the cycle-life-time. This slight degradation took place due to the grain growth came off in the Mg/Zr2Ni grains. (C) 2015 Elsevier Ltd. All rights reserved.