A series of poly(acetoxystyrene-co-octavinyl-polyhedral oligomeric silsesquioxane) (PAS-POSS) hybrid nanocomposites were synthesized by free radical polymerization and characterized by FTIR, H-1 NMR, GPC, DSC, and TGA technologies. The POSS contents in these nanocomposites were calculated on the basis of FTIR data. The results show that POSS content can be controlled by varying the POSS feed ratios. DSC and TGA measurements reveal that the incorporation of POSS into polymers can improve the thermal properties of polymeric materials. The FTIR spectra were employed to explaln the T-g improvement mechanism. POSS moieties influence the T-g of nanocomposite in two ways. At relatively low POSS contents, POSS plays a role of inert diluent to reduce T-g. With the increase of POSS contents, both the aggregation of POSS nanoparticles and the dipole-dipole interaction between POSS and PAS molecules contribute to the increase of T-g.