Abstract: Binary nanocomposites, such as poly(methyl methacrylate)/ Na+montmorillonite (PMMA/MMT) and poly(methyl methacrylate)/silica sol (PMMA/SiS), and ternary nanocomposites, PMMA/Na+montmorillonite /silica sol (PMMA/MMT/SiS), were prepared via insitu soapfree emulsion polymerization. The gel permeation chromatography, smallangle X-ray diffractometry, differential scanning colorimeter, thermogravimetric analysis and tensile tests were carried out to investigate the effect of type and fraction of nanoparticles on the polymer molecular weight and distribution, thermal stability and tensile strength of the nanocomposites. The results show that the molecular weight distribution is broadened by introducing the nanoparticles. At a certain nanoparticle fraction, the ternary nanocomposite is more efficient in improving thermal stability and tensile strength of PMMA than binary nanocomposites.