Effect of addition of magnesium nitrate hexahydrate on the reactivity of soft-burned magnesia was experimentally investigated. We found that the reactivity of magnesite which passed 280 mesh was 0.070, and the reactivity of that was increased by calcining. When magnesium nitrate hexahydrate was added to magnesite, the soft-burned magnesia with a high reactivity was produced in heating for a short time.