To investigate the effect of microstructure on the better de-/hydrogenation property of Mg-based alloy, hypo eutectic Mg-8Ni (at. %) alloy and hyper-eutectic Mg-15Ni alloy are prepared by metallurgy method. The phase constitutions and microstructures are characterized by XRD and SEM/EDS. Mg-8/15Ni alloy is composed of primary Mg/Mg2Ni and eutectic Mg-Mg2Ni. In isothermal sorption test, Mg-15Ni alloy shows preferable activation performance and faster de-/hydrogenation rates than Mg-8Ni alloy. The respective hydrogen uptake capacity in 165min is 5.62 wt% and 5.76 wt% H-2 at 300 degrees C 3 MPa. Intersections of Mg-Mg2Ni eutectic phase boundaries with particle surface provide excellent sites and paths for the dissociation and permeation of hydrogen. The de-/hydrogenation enthalpy and entropy values are determined by PCI measurement. Based on the DSC curves at different heating rates, the desorption behavior of Mg-8/15Ni hydride is revealed and the respective activation energy is calculated to be 134.67 kJ mol(-1) and 88.34 kJ mol(-1) H-2 by Kissinger method. Synergic dehydrogenation occurs in eutectic MgH2-Mg2NiH4, which facilitates the primary MgH2 in Mg-8Ni hydride to decompose at a lower temperature. The rapid H diffusion and synergic effect in eutectic MgH2-Mg2NiH4 collectively contribute to the lower dehydrogenation energy barrier of Mg-15Ni hydride.