We have performed ab initio molecular dynamics simulations on proposed hydrogen storage system of lithium hydride (LiH) and ammonia (NH 3 ), i.e., LiH+NH 3 ↔LiNH 2 +H 2 to clarify the microscopic mechanism of the hydrogen desorption for a system of one Li 2 H 2 cluster and one NH 3 molecule. We have shown, for the first time, the dynamical reaction process for the H 2 molecule formation from two H atoms, one of which is from the LiH cluster and the other is from the NH 3 , and each H atom of H 2 is not equivalent both in geometry and in charge during the reaction. The reaction model, regarding this H 2 desorption as the only reaction process of hydrogen desorption from LiH and NH 3 , can explain the recent experiment for hydrogen desorption from LiD and LiNH 2 by including the effects of reverse reactions. This model gives the ratios H 2 :HD:D 2 and Li 2 NH:Li 2 ND close to those obtained by the experiment, and desorption profiles of H 2 , HD and D 2 which qualitatively reproduce the characteristics of experimental profiles.