The Mg-10.2 at.% V nanoparticles are prepared by hydrogen plasma-metal reaction (HPMR) method. These nanoparticles are made of Mg, VH2 and a small amount of MgH2. The Mg nanoparticles are hexagonal in shape with the particle size in the range of 50-150 nm. The VH2 nanoparticles are spherical in shape with the particle size around 10 nm, and disperse on the surface of the Mg nanoparticles. After the hydrogen absorption, the mean particle size of MgH2 decreases to 60 nm, while the V nanoparticles are still about 10 nm. The Mg-V composite nanoparticles can absorb 3.8 wt.% hydrogen in less than 30 min at 473 K and accomplish a high hydrogen storage capacity of 5.0 wt.% in less than 5 min at 623 K. They can release 4.0 wt.% hydrogen in less than 15 min at 573 K. The catalytic effect of the V nanoparticles and the nanostructure and the low oxide content of the Mg particles promote the hydrogen sorption process with the low hydrogen absorption activation energy of 71.2 kJ mol(-1). (C) 2011 Elsevier B.V. All rights reserved.