Lithium magnesium thio-phosphate, Li4-2xMgxP2S6, was synthesized, and its crystal structure and ionic conduction were investigated. The structure is CdI2 type, where two thirds of the Cd sites are occupied by Mg2+ and Li+ ions with an occupancy ratio of x/(2-x), and the residual Li+ ions reside in the van der Waals gap in the CdI2 structure. (P4+)(2) paired-ions occupy the octahedral sites in the van der Waals gap as well as the remaining Cd sites with a statistical distribution. With the increase in the magnesium composition, the occupancy for the former sites decreases. As a result, the vacancies were generated in the alternate lithium layers, where Li+ ions become mobile.