Magnesium–tantalum ternary hydrides with an FCC structure were prepared by an ultra high-pressure technique. The FCC Mg–Ta hydride phases with the Ca 7 Ge-type super-lattice structure were formed at 4 and 8GPa. Another new phase, Mg 3 TaH y , was also verified by XRD and SEM in the specimen prepared at 8GPa. From the temperature-programmed desorption (TPD) measurements with a heating rate of 10K/min under vacuum, the FCC hydride exhibited hydrogen desorption of ca. 3.1mass% at an onset temperature of 583K. When the FCC hydride phase coexisted with Mg 3 TaH y , simultaneous decomposition occurred and the hydrogen-desorption temperature was lowered to 543K. The hydrogen-desorption temperatures of the Mg–Ta FCC hydrides were 130–170K lower than that of MgH 2 .