Montmorillonite(MMT) modified with sodium silicate can change the arrangement of its layers from edge-face and edge-edge to face-face. With the fine dispersion of the modified MMT in water, the cation-exchange reaction was carried out with cetyltrimethyl ammonium bromide(CTAB) to obtain organo-montmorillonite(OMMT). As OMMT was uniformly dispersed in methyl methacrylate(MMA) monomer, PMMA/OMMT nanocomposites were formed via a common emulsion polymerization. The products were extracted with hot acetone and characterized by FTIR, molecular weight, X-ray diffraction(XRD), transmission electron microscopy(TEM), DSC and TGA. These results show that most of the OMMT layers have been exfoliated, while the thermal stability is increased obviously. By means of FTIR spectral analysis, the ratios of the macromolecular radicals′ termination of disproportionation patterns to combination are increased with the addition of OMMT. This result further confirms the increase of the thermal degradation temperatures and glass transition temperatures of the PMMA/OMMT nanocomposites.