Mg(NH2)2-LiH system which have the properties of reversible hydrogenation and dehydrogenation is one of the promising candidates for new hydrogen storage materials. For understanding of the reversible reaction mechanism, we investigated the crystal structure changes in 3Mg(NH 2)2-12LiH system using the pressure-composition (p-c) isotherm measurement and synchrotron X-ray diffraction. The sample was prepared by the hydrogenation of Mg3N2 + 4Li3N. At the several dehydrogenation stages of the p-c isotherm measurement at temperature 523 K, the sample was taken out and X-ray diffraction measurement was performed. By the amount of desorbed hydrogen, the reaction was expressed as the following formula, Mg(NH2)2 + 4LiH - LixMg(NH 2)2-x(NH)x + (4-x)LiH + xH2 (x = 0-2). The crystal structures of LixMg(NH2(NH)x, similar to CaF2-type one, formed during the dehydrogenation reaction were determined by Rietveld analysis. As a result, it is considered that the dehydrogenation process might relate to the diffusion of Li+ ion in cation sites of Mg(NH2)2.