Abstract: A process to prepare high purity Mg O with brine and industrial ammonia gas was introduced. 4MgCO₃·Mg (OH) ₂·4H₂O of orthorhombic crystal, prepared under the selected condition, was calcined at 600℃, 700℃, 800℃, and 900℃, respectively. Then, the products were characterized. The results suggested that with the increasing of calcination temperature, the crystal's prefered growing orientation remained the same, while the crystallinity and the size increased, and the micro-structures changed. The calcination temperature higher than 700℃ made no futher difference in the purity of the products.