Magnesium hydroxide nanoneedles modified by oleic acid [OA-Mg(OH)(2)] were synthesized via alkaline injection into magnesium chloride solution in the presence of oleic acid (OA). The magnesium hydroxide/polystyrene nanocomposite [Mg(OH)(2)/PS] was then prepared by soapless emulsion polymerization technique in an aqueous suspension. The Fourier transform infrared (FT-IR) analysis shows that the polystyrene was covalently grafted onto the surfaces of the Mg(OH)(2) nanoneedles [OA-Mg(OH)(2)]. The dispersion of Mg(OH)(2) nanoneedles in polystyrene is uniform as observed by transmission electron microscope (TEM). The grafting percentage was determined by means of elemental analysis (EA). The thermal behavior determined by the differential scanning calorimetry (DSC) indicated that Mg(OH)(2)/PS had a higher glass transition temperature than PS matrix, and it can be concluded that Mg(OH)(2) could retard the degradation of PS based on the data of thermal gravimetric analysis (TGA) and differential thermoanalysis (DTA).