Reactive ball milling (REM) technique was employed to synthesize ultrafine powders of MgH2, using high energy ball mill operated at room temperature under 50 bar of a hydrogen gas atmosphere. The MgH2 powders obtained after 200 h of continuous RBM time composed of beta and gamma phases. The powders possessed nanocrystalline characteristics with an average grain of about 10 nm in diameter. The time required for complete hydrogen absorption and desorption measured at 250 degrees C was 500 s and 2500 s, respectively. In order to improve the hydrogenation/dehydrogenation kinetics of as synthesized MgH2 powders, three different types of nanocatalysts (metallic Ni, Nb2O5 and (Ni)(x)/(Nb2O5)(y)) were utilized with different weight percentages and independently ball milled with the MgH2 powders for 50 h under 50 bar of a hydrogen gas atmosphere. The results showed that the prepared nanocomposite MgH2/5Ni/5Nb(2)O(5) powders possessed superior hydrogenation/dehydrogenation characteristics, indexed by low values of activation energy for beta-phase (68 kJ/mol) and gamma-phase (74 kJ/mol). This nanocomposite system showed excellent hydrogenation/dehydrogenization behavior, indexed by the short time required to uptake (41 s) and release (121 s) of 5 wt% H-2 at 250 degrees C. At this temperature the synthesized nanocomposite powders possessed excellent absorption/desorption cyclability of 180 complete cycles. No serious degradation on the hydrogen storage capacity could be detected and the system exhibited nearly constant absorption and desorption values of +5.46 and -5.46 wt% H-2, respectively. Copyright (c) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.