The poly(styrene-co-octavinyl-polyhedral oligomeric silsesquioxane) (PS-POSS) organic-inorganic hybrid nanocomposites containing various percent of POSS were prepared via one-step free radical polymerization and characterized by FTIR, high-resolution H-1 NMR, Si-29 NMR, GPC, DSC, and TGA technologies. The POSS contents in these nanocomposites were determined using FTIR calibration curve. The result shows that the POSS contents in nanocomposites can be tailored by varying the POSS feed ratios. On the basis of the POSS contents in the nanocomposites and the 1H NMR spectra, the number of reacted vinyl groups of each octavinyl-POSS macromonomer were calculated to be 6-8. DSC and TGA measurements indicate that the incorporation of POSS into PS homopolymer can apparently improve the thermal properties of the polymeric materials. The dramatic T-g and T-dec increases are mainly due to the formation of star and low cross-linking structure of the nanocomposites, where POSS cores behave as the joint points and hinder the motion and degradation of the polymeric chains. (c) 2007 Wiley Periodicals, Inc.