Organically modified montmorillonite ($C_8PPh-MMT$) was obtained using the ion exchange reaction between $Na^+-montmorillonite;(Na^+-MMT)$ and 1-octenyltriphenyl phosphonium chloride ($C_8PPh-Cl^-$). Polystyrene nanocomposites were then prepared by in-situ free-radical polymerization of the styrene containing intercalated $C_8PPh-MMT$. The resulting polystyrene hybrids with various organoclay contents were investigated with FT-IR, which confirmed that PS hybrids were successfully prepared via the reaction of styrene monomer in the interlayers of the clays. The variations of the thermal behaviors of the hybrids with increases in the organoclay content from 0 to 8 wt% were determined. The glass transition temperatures ($T_g$) and initial thermal degradation temperatures ($T_D^i$) of the PS hybrids were found to increase linearly with increases in the organoclay loading. Regardless of the organoclay content of the hybrids, the clay was found to be dispersed homogeneously in the matrix polymer. This is direct evidence that the PS hybrids formed nanocomposites. This result was confirmed with XRD and TEM.