High-purity anhydrous magnesium chloride was prepared by using magnesia and ammonium chloride as reactants, alumina as covering agent. The process was characterized by X-ray diffraction and thermogravimetric and differential scanning calorimetry analysis. The reaction mechanism involved in the process was proposed and discussed. In addition, the effects of molar ratio of ammonium chloride to magnesia, thickness of the covering agent (alumina), reaction temperature, and reaction time on the purity of anhydrous magnesium chloride were investigated. The content of magnesia in anhydrous magnesium chloride was achieved 0.014% under the optimum conditions: ammonium chloride and magnesia with the molar ratio of NH4Cl:MgO = 5:1 were mixed evenly. Alumina with the thickness of 1.1 cm was used as a covering agent. The mixture was maintained at 450 degrees C for 1.5 h for the complete reaction, and then calcined at 700 degrees C for 0.5 h to obtain well-sintered anhydrous magnesium chloride.