Magnesia was formerly produced by calcining magnesite (MgCO3) or brucite (Mg(OH)2) from Manchuria and Korea. But these raw materials are now entirely diffidult to obtain from these countries and our country itself. So that, magnesia or magnesium hydroxide, which is very important for various uses, e. g., basic magnesian refractories, magnesia cement. magnesium salt chemicals, etc., must be obtained directly from sea weter, or its evaporated brine or bittern from salt making plant.In the pretent paper, vanious processes of makiug magnesium hydroxide or magnesia from the ore of magnesite and brucite, or sea water were fully discussed, and the proeesses of magnsia making by treating ses water, brine or bittern with calcined dolomite or lime from limestone calcining, were determined to be the best process in the present state of no production of magnesite or brucite, and these processes were preliminary tested with full discussion on the treating conditions and the chemical compositions, especially the relations between various conditions of treating method and the amount of intermixed lime to the principal component of magnesia. The best quality of magnesia was obtained by mixing dilute solution (Be 15-20°) of bittern or brine with slaked solution of calcined dolomite (700-800°C), in the following two steps: (1) small amount of calcined dolomite equivalent to SO4 radical for precipitation of CaSO4⋅2H2O, and then (2) calculated sufficient amount of calcined dolomite equivalent to Mg radical for precipitation of Mg(OH)2. Magnesium hydroxide or magnesia containing small amount of lime was also obtained by using dilute solution of quick lime or fresh slaked lime, by the same way in the two steps as above described.