A series of poly(methyl methacrylate) (PMMA) containing octavinyl-polyhedral oligomeric silsesquioxane (OV-POSS) nanocomposites were synthesized by solution polymerization. The products were characterized by FTIR, H-1 NMR, GPC, TEM, DSC and TGA. The actual contents of OV-POSS in the obtained products and the reaction degree of the vinyl groups in the POSS were calculated on the basis of FTIR, TGA and H-1 NMR data respectively. The DSC and TGA results indicate that the incorporation of POSS molecules could improve the thermal properties of PMMA nanocomposites significantly. The glass transition temperature (T-g) and thermal decomposition temperature (T-dec1) of the nanocomposite with 12.27 wt % of OV-POSS were increased by 23 degrees C and 93 degrees C correspondingly. In our experiment, the improved thermal properties were largely attributed to the nanoreinforcement effect of POSS cages and the formation of star-shaped structures with cubic silsesquioxane core.