Polystyrene-clay nanocomposites were synthesized via emulsion polymerization of styrene in the presence of montmorillonite particles and a reactive cationic surfactant as the emulsifier. The reactive surfactant, vinylbenzyldodecyldimethylammonium chloride (VDAC), was synthesized by the quaternization reaction of vinylbenzyl chloride with dimethyldodecylamine. Partially exfoliated nanocomposites were achieved as revealed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Polystyrene-clay nanocomposites prepared by emulsion polymerization have higher glass transition temperature and dynamic modulus than pure polystyrene.