In this paper, the performance tests on Mg + 30% MmNi(4) based thermal energy storage device is presented. Experiments were carried out at different supply pressures (10-30 bar) and absorption temperatures (120-150 degrees C). The effects of hydrogen supply pressure and absorption temperature on the amount of hydrogen/heat stored and thermal energy storage coefficient are presented. The maximum hydrogen storage capacity of 2.5wt% is reported at the operating conditions of 20 bar supply pressure and 150 degrees C absorption temperature. For a given absorption temperature of 150 degrees C, the thermal energy storage coefficient is found to increase from 0.5 at 10 bar to 0.74 at 30 bar supply pressure. For the given operating conditions of 20 bar supply pressure and 150 degrees C absorption temperature, the maximum amount of heat stored is about 0.714 MJ/kg and the corresponding thermal energy storage coefficient is 0.74. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.