The heat produced by a solar receiver during on-sun operation can be employed to drive the endothermic reduction reaction of Co3O4 to CoO; then the consumed thermal energy can be recovered completely by the exothermic reverse oxidation reaction of CoO to Co3O4 which can take place during off-sun operation. In this research, the effect of mechanical activation duration (1, 2, 4, 8, and 16 h) on thermal energy storage by Co3O4/CoO redox pair was investigated. It was found that increasing the mechanical activation duration increases the sintering and particle size of the cobalt oxide powder after one cycle redox, and subsequently the thermal energy storage properties are declined. The weight loss was about 4-5 wt.% for samples heated by 1, 3, and 5 degrees C/min, while it was about 2 wt.% for 10 degrees C/min heating rate and less than 1 wt.% for 15 degrees C/min heating rate. The comparison of cycleability of as-received and 1 h mechanical activated cobalt oxide showed that mechanical activation weakens the cycleability of redox reactions of cobalt oxide. The as-received cobalt oxide cycleability continued up to three cycles, although the reduction and oxidation capacities gradually declined. The cycleability of 1 h mechanical activated sample entirely diminished after two cycles. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.