A novel reversible addition-fragmentation transfer (RAFT) agent, 10-carboxylic acid-10-dithiobenzoate-decyltrimethylammonium bromide (CDDA), was synthesized and intercalated into montmorillonite (MMT). Successively, the CDDA-intercalated MMT was used Lis RAFT agent in the in Situ RAFT polymerization for preparation of the polystyrene/MMT nanocomposites. After separation of MMT, the polymers obtained have predictable molecular weight and narrow polydispersity. XRD spectra and TEM images of the nanocomposites demonstrated exfoliated structure. Thermal stability of the composites has been noticeably improved.