Abstract: The organic nano-montmorillonite (OMMT) /poly ( styrene-co-divinylbenzene , St ) composites were prepared by suspension polymerization. OMMT was prepared by the ionexchange method between sodium montmorillonite (SMMT) and cetylt rimethyl ammonium bromide (CTAB) in an aqueous solution. The character and structure of OMMT were revealed by XRD and FTIR. The microstructure of composites was characterized by XRD, TEM, SEM and FTIR. The thermal stability of composites was studied by TG. The effect of the amount of OMMT and divinylbenzene (DVB) on the thermal stability of composites was discussed. The results indicate that the maximum interlayer spacing is 1. 48 nm for SMMT, 2. 85 nm for OMMT and about 4. 0 nm for OMMT/polymer. The maximum decomposed temperatures are 399 ℃ for the pure polystyrene and 437 ℃ for composites with montmorillonite mass f raction of 5 % and DVB content of 10 %. The crosslinked composites exhibit drastic improvements in hermal stability under the experimental conditions.