Micron magnesium hydroxide [Mg(OH) 2 ] was modified by means of ultrasonic method with stearic acid (SA) as modifier in water. The Fourier transform infrared spectroscopy (FT-IR) and element analysis showed all SA was bonded upon the surface of the Mg(OH) 2 forming a coating layer and no free SA was detected after the modifying process. The thickness of coating SA on the Mg(OH) 2 was determined by X-ray photoelectron spectroscopy (XPS). Compared with the unmodified Mg(OH) 2 , the modified Mg(OH) 2 had better dispersion property in xylene, slower sedimentation velocity of dilute suspension in xylene and lower viscosity of suspension in paraffin liquid. The results showed that the modified Mg(OH) 2 could be dispersed much better than the unmodified Mg(OH) 2 in organic phase.