Highly crumpled graphene nanosheets (GNS) with a BET surface area as high as 1159 m(2) g(-1) was fabricated by a thermal exfoliation method. A systematic investigation was performed on the hydrogen sorption properties of MgH(2)-5 wt% GNS nanocomposites acquired by ball-milling. It was found that the as-synthesized GNS exhibited a superior catalytic effect on hydrogenation/dehydrogenation of MgH(2). Differential Scanning Calorimetry (DSC) and isothermal hydrogenation/dehydrogenation measurements indicated that both hydrogen sorption capacity and dehydrogenation/hydrogenation kinetics of the composites improved with increasing milling time. The composites MgH(2)-GNS milled for 20 h can absorb 6.6 wt% H(2) within 1 min at 300 °C and 6.3 wt% within 40 min at 200 °C, even at 150 °C, it can also absorb 6.0 wt% H(2) within 180 min. It was also demonstrated that MgH(2)-GNS-20 h could release 6.1 wt% H(2) at 300 °C within 40 min. In addition, microstructure measurements based on XRD, SEM, TEM as well as Raman spectra revealed that the grain size of thus-prepared MgH(2)-GNS nanocomposites decreased with increasing milling time, moreover, the graphene layers were broken into smaller graphene nanosheets in a disordered and irregular manner during milling. It was confirmed that these smaller graphene nanosheets on the composite surface, providing more edge sites and hydrogen diffusion channels, prevented the nanograins from sintering and agglomerating, thus, leading to promotion of the hydrogenation/dehydrogenation kinetics of MgH(2).