Abstract: Mg(OH)2 colloid was precipitated at room temperature using MgCl2 and NH3·H2O as precursors. Then, the superfine high-purity Mg(OH)2 particles were obtained by hydrothermal treatment of Mg(OH)2 precipitate in different reaction media. The crystal phase, morphology and particles size of the Mg(OH)2 particles obtained were characterized with XRD and SEM. The results indicated that different reaction media play an important role in the formation of Mg(OH)2 crystals, and the NaOH solution, ethylenediamine or ethanol solution can significantly improve the crystal morphology and dispersion of Mg(OH)2 crystals. The influence of reaction media on the morphology of Mg(OH)2 crystals under hydrothermal conditions was discussed preliminarily.