Electronic structure and the total energy of the Mg(NH 2 ) 2 were calculated using first principle theory. The bonding characteristics and decomposition mechanism of the Mg(NH 2 ) 2 were clarified based on the electronic structure and the total energies. The bonding interactions of the Mg atoms with the two [NH 2 ] ligands are slightly different, while it shows a significant difference in the bonding interactions between the N and the H atoms within the [NH 2 ] ligands. The weakest bond is the N2–H2 bond in the [NH 2 ](2) ligand. A decomposition mechanism of the Mg(NH 2 ) 2 was proposed based on the bonding characteristics. The decomposition of the Mg(NH 2 ) 2 is performed by two steps. First H + cations decompose from the [NH 2 ] ligands due to their weaker bonds with the matrix, and then [NH 2 ] − anions decompose. The H + cations and [NH 2 ] − anions therefore react each other to generate NH 3 . For the Mg(NH 2 ) 2 +LiH systems, it is most likely that the Mg(NH 2 ) decomposes to MgNH, H + cation, and [NH 2 ] − anion first, and then the released H + cation and [NH 2 ] − anion either react each other to form NH 3 and then reacts with LiH, or directly react with Li + cation and H − anion if LiH is decomposed. Both of the reactions generate the LiNH 2 and the H 2 . And the LiNH 2 further mixes with MgNH to form the LiMgN 2 H 3 . The is the first step of a multi-step dehydrogenation process of the Mg(NH 2 ) 2 –LiH system [Isobe S, Ichikawa T, Leng H, Fujii H, Kojima Y. Hydrogen desorption processes in Li–Mg–N–H systems. J Phys Chem Solids 2008;69:22234.].