Mg 2 NiH 4 , with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg 2 NiH 4 , and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg 2 Ni and a destabilization of the hydride Mg 2 NiH 4 , which significantly alters the hydrogen desorption enthalpy DH des for the reaction Mg 2 NiH 4 / Mg 2 Ni รพ 2H 2 . A desirable enthalpy value of w0.4 eV/H 2 for application can be obtained for a doping level of x ! 0.35 in Mg 2รx Al x Ni alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg 2 Ni and Mg 2 NiH 4 exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg 2 NiH 4 is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg 2 Ni and Mg 3 AlNi 2 as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg 2 NiH 4 can be attributed to the weakened MgeNi and NieH interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.