First-principle density functional theory calculations were used to investigate the electronic structure and mechanism of the LiH + NH 3 → LiNH 2 + H 2 reaction. Along the reaction pathway, intermediate complexes HLi…NH 3 and LiNH 2 …H 2 and a transition state can be found. The N-2p electron in the highest occupied molecular orbital (HOMO) of NH 3 transfers to the Li-2s orbital in lowest unoccupied molecular orbital (LUMO) of LiH and forms the initial state HLi…NH 3 . In the transition state, H1 of LiH and H2 of NH 3 turn toward each other, resulting in the formation of a H 2 bond. From the transition state to the final state, the geometric configuration changes from C s to C 2v , and the improvement of geometric configuration symmetry results in a decrease in the energy gap between HOMO and LUMO. The LiH + NH 3 → LiNH 2 + H 2 reaction is exothermic.