To prepare the polystyrene (PS)-clay nanocomposites via an in situ emulsion polymerization, a clay predispersion method, i.e. dispersing the organic clay in the emulsifier solution by the assistance of ultrasonic, was proposed in this study. The conventional method, pre-dispersing the organic clay into the monomer, was also presented for the comparison. The morphology analysis based on the X-ray Deflection (XRD) and Transmission Electronic Microscopy (TEM) results suggested that the more uniform clay dispersion in the final nanocomposites could be achieved through the new method. The inorganic clay (Na-MMT) and two organic clays (C-18-MMT and VC18-MMT) synthesized by exchanging inorganic cations with the trimethyloctadecyl ammonium chloride (OTAC) and the vinylbenzyl-dimethyloctadecyl ammoniun chloride (VOAC) were chosen to investigate the influence of the clay surface modification on the properties of nanocomposites. The Dynamic Mechanical Analysis (DMA) results showed the storage modulus G's of the nanocomposites had different enhancements over that of the pure PS, especially when the temperature approached the glass transition temperature (T-g). The T(g)s of the nanocomposites, however, varied with the microstructure and the interactions between the polymer and the clay layers. The Na-MMT and VC18-MMT increased the T-g, while the T(g)s of PS/C-18-MMT nanocomposites were slightly lower than that of the pure PS. POLYM. ENG. SCI., 49:1937-1944, 2009. (C) 2009 Society of Plastics Engineers