The electronic structures, dehydrogenation properties and diffusion path of an H atom in pure and transition metal substituted Mg(BH4)(2)center dot 2NH(3) were investigated using the first-principles calculations. The occupation energies of dopants indicate that substituting Ti for an Mg1 atom is the easiest, while Ni and Nb substitutions are somewhat difficult. The electronic density of states and the topological analysis of electron density reveal the covalent characteristics of the N-H and B-H bonds and the weak ionic interaction between M (the metal atoms) and the NH3 and BH4 groups. The substitutions of Ti, Ni and Nb increase the interaction between metal and N atoms, which stabilize the NH3 groups and inhibit the release of ammonia during dehydrogenation. The hydrogen removal energies indicate that Ti, Ni or Nb doping weakens the interactions between B and H atoms, thereby facilitating the dissociation of B-H bonds in Mg(BH4)(2)center dot 2NH(3). The minimum energy paths of H diffusion demonstrate that the substitutions can reduce the energy barrier and thus favor H diffusion in the bulk phase. Thus, substitution is an effective technique for improving the hydrogenation/dehydrogenation performance of Mi(BH4)(2)center dot 2NH(3) hydrogen storage material. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.