This research focuses on dehydration / hydration of magnesium hydroxide as a chemical heat storage material. Previous studies have reported that the use of additives in magnesium hydroxide improved the dehydration / hydration reactivity. However, additives used in previous studies have had problems in terms of environmental impact and cost. Therefore, the purpose of this study is to search for safe and inexpensive additives. We have selected citrate compounds as an inexpensive and safe additive. The effect of the additive was verified by measuring the dehydration / hydration reaction of magnesium hydroxide using a thermogravimetric instrument. Furthermore, XRD was used for sample characterization. As a result, the most improved reactivity was confirmed in the sample using sodium citrate as an additive. SC5 (molar ratio, magnesium hydroxide : sodium citrate dihydrate = 100 : 5) decreased the dehydration peak temperature by about 31ºC compared to pure magnesium hydroxide. Sodium citrate dihydrate was found to undergo thermal degradation during sample heating. Then, when the repeated reaction test was implemented, the improvement of the dehydration rate after the 2nd time was confirmed. These results indicate that the product of thermal decomposition of sodium citrate dihydrate is effective as an additive.