Abstract: Surface modified nano-sized mgnesium hydroxide was prepared by direct precipitation method with magnesium nitrate hexahydrate and sodium hydroxide as raw materials,and sodium fatty alcohol polyoxyethylene ether carboxylate as modifying agent.The optimum dosage of modifying agent was 0.2% of the mass of magnesium hydroxide,which was confirmed by sediment volume experiment.Modified and unmodified samples were characterized by fourier transform infrared spectroscopy (FT-IR),X-ray diffraction (XRD),transmission electron microscope (TEM),and tap density test.Results indicated that sodium fatty alcohol polyoxyethylene ether carboxylate was adsorbed onto the surface of magnesium hydroxide nano-particles changing the hydrophilic surface into hydrophobic surface and improving the dispersion stability in organic medium. Compared to unmodified magnesium hydroxide,the main XRD diffraction peak of nano-sized magnesium hydroxide was strengthened showing better crystalline,lower agglomeration,and smaller tap density after modified.