Poly(methyl methacrylate) (PMMA)/SiO2 nanocomposites were prepared by in situ suspension polymerization. Two types of modified methods were used to modify nano-SiO2: one was modification by gamma-methacyloxypropyl trimethoxy silane (KH570) and lauryl alcohol (12COH) while the other was grafting PMMA onto the surface of KH570 treated SiO2. Transmission electron microscopy (TEM) and Fourier transformed infrared (FTIR) were used to characterize the structures of the nanocomposites. The influence of synthetic conditions, for instance, surface modification, initial SiO2 contents and reaction temperature, on the microsphere's size and molecular weight of the extracted PMMA were studied by gel permeation chromatograph (GPC) and optical microscopy (OM) in details. Thermal property of the nanocomposites was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results indicate that the presence and content Of SiO2 have a vital effect on the shape and size of the nanocomposite microspheres, as well as molecular weight of the extracted PMMA. Grafting polymer to the surface Of SiO2 is an effective way for the purpose of effective in situ suspension polymerization. Compared to pure PMMA, the thermal properties of the nanocomposites were improved. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1975-1981, 2010