Mg+10%Ni2P composite (mass fraction, similarly hereinafter) and pure magnesium samples were prepared by reactive ball-milling under hydrogen atmosphere, and their phase-structure and hydrogen storage behavior were investigated systemically. The results show that the hydrogen absorption/desorption kinetics of pure Mg are markedly improved by adding 10% Ni2P. Furthermore, the catalytically enhanced kinetic property is found to persist in the de-/hydrogenation cycles. The size of crystal grains of the composite is reduced by prolonging the ball-milling time. Ni2P is effective in inhibiting the aggregation of Mg during the de-/hydrogenation processes. The doping of Ni2P can lower the dehydriding temperatures of the hydriding Mg-Ni2P composites.