The effect of high pressure (6 GPa) and high temperature (600 o C) on the Mg 2 NiH 4 -H system has been investigated by using thermogravimetry, differential thermal analyses, and X-ray diffraction. The high-pressure treated Mg 2 NiH x (HP-Mg 2 NiH x ) can desorb 5.2 wt% of hydrogen from room temperature to 400 o C. This value is about 1.6 wt% higher than that of standard Mg 2 NiH 4 . The hydrogen stored in HP-Mg 2 NiH x is released at quite a low temperature owing to the unstability of the sample that may have a changed distribution of hydrogen site energy. Results indicate that metal-hydrido complex Mg 2 NiH 6 with a cubic structure is likely to exist.