The decomposition mechanisms of Mg(BH 4 ) 2 ·2NH 3 and LiMg(BH 4 ) 3 ·2NH 3 were studied by using density functional theory calculations. Compared to that of Mg(BH 4 ) 2 ·2NH 3 , the incorporation of LiBH 4 with the formation of LiMg(BH 4 ) 3 ·2NH 3 slightly increased Bader charges of B atoms, meanwhile it decreased Bader charges of N atoms. Mg(BH 4 ) 2 ·2NH 3 shows a low ammonia vacancy diffusion barrier, but relatively high ammonia vacancy formation energy, which lead to a low concentration of NH 3 vacancies and limit NH 3 transportation. In contrast to that of Mg(BH 4 ) 2 ·2NH 3 , LiMg(BH 4 ) 3 ·2NH 3 has a relatively high ammonia vacancy formation energy and diffusion barrier, which suppresses ammonia release. The incorporation of LiBH 4 and Mg(BH 4 ) 2 ·2NH 3 does not decrease but increases the hydrogen formation barrier of LiMg(BH 4 ) 3 ·2NH 3 , resulting in a slight increase in the dehydrogenation peak temperature, consistent with experimental results.