Mg2NiH4, 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 Mg2NiH4, 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 Mg2Ni and a destabilization of the hydride Mg2NiH4, which significantly alters the hydrogen desorption enthalpy Delta H-des for the reaction Mg2NiH4 --> Mg2Ni + 2H(2). A desirable enthalpy value of 0.4 eV/H-2 for application can be obtained for a doping level of x >= 0.35 in Mg2-xAlxNi alloy. The si:ability calculations by considering possible decompositions indicate that the Al-doped Mg2Ni and Mg2NiH4 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 Mg2NiH4 is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg2Ni and Mg3AlNi2 as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg2NiH4 can be attributed to the weakened Mg-Ni and Ni-H 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. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.