The hydrogen storage properties of nanocrystalline MgH 2 obtained by ball milling were studied. In particular, ball milling of MgH 2 with cyclohexane resulted in improvement of the desorption properties. The desorption temperatures of the nanocrystalline MgH 2 were strongly dependent upon the rehydrogenation temperatures; the sample rehydrogenated at 290 K showed a lower desorption temperature by about 90 K than that rehydrogenated at 453 K. Such a desorption behavior is associated with lattice defects such as dislocation and vacancy introduced during rehydrogenation. Upon ball milling of nanocrystalline MgH 2 with Sn, the properties of MgH 2 were significantly improved for hydrogen storage. Thermal desorption spectrometry (TDS), thermogravimetry (TG) and differential scanning calorimeter (DSC) measurements suggest the formation of MgH 2/Sn nanocomposite as a result of ball milling of MgH 2 and Sn with cyclohexane. The MgH 2/Sn nanocomposite contained two types of hydrogen species; the one was hydrogen in the newly formed MgH 2/Sn nanocomposite and the other hydrogen derived from MgH 2 remaining in MgH 2/Sn. The properties of the MgH 2/Sn nanocomposite were further discussed in comparison with those of the physical mixture of MgH 2 and Sn.