Abstract: The nano-sized magnesium hydroxide powder was prepared by direct precipitation method with brine and sodium hydroxide as raw materials under the assistance of surfactants.The influences of different conditions，such as reaction temperature，reactant amount-of-substance ratio，surfactant kinds，and drying and dropping methods，on particle size of the obtained powder were investigated.The powders were characterized by XRD and SEM.The products′ thermal stability was analyzed by TG-DTA，and the products′particle size and distribution were tested with Laser particle size analyzer.Results showed:flaky nano-magnesium hydroxide about 10 nm in width with good flow-ability was obtained by azeotropic distillation with n-butanol and with PEG200 as surfactant，under the suitable reaction temperature and the amount-of-substance ratio.

Key words: nano-sized magnesium hydroxide, brine-sodium hydroxide, direct precipitation method, azeotropic distillation