A systematic investigation was performed on the hydrogen storage properties of composites which were prepared by ball milling MgH 2 with different amounts of LaH 3 and the catalysis mechanism of La hydride on MgH 2 was reported in this paper. Pressure–Composition–Temperature (P–C–T) curves showed that the reversible hydrogen storage capacity of MgH 2 +20wt.% LaH 3 composite was 5.1wt.% at 548K, while the pure MgH 2 hardly released any H 2 under the same conditions. The addition of LaH 3 also significantly improved the hydriding/dehydriding kinetics, and led to the rate-controlling steps of MgH 2 becoming altered from a three-dimensional interfacial reaction to a one-dimensional diffusion process. The XRD pattern indicated that the LaH 3 phase partially transformed to LaH 2.3 phase during the dehydriding process. TEM micrograph images revealed that the LaH 2.3 phase was distributed homogeneously throughout the Mg phase and that the Mg crystals were coated with LaH 2.3 crystals in the matrix. This microstructure exhibited an obvious volume contraction and resulted in a distinct strain of MgH 2 when the LaH 3 phase released H 2 . DSC curves proved that the addition of LaH 3 could decrease the temperature at which the onset of the dehydrogenation of MgH 2 occurred by approximately 20.4K.