Incorporation of polyhedral oligomeric silsesquioxane (POSS) molecules into polymer matrix resulted in increased used and decomposition temperatures. Polymethylsiloxane (PMS)/MonoSilanolIsobutyl-POSS hybrid copolymers containing various proportions of MonoSilanolIsobutyl-POSS were prepared. The structures and thermal properties of the obtained products were characterized with Gel Permeation Chromatography (GPC), Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTG) and X-ray photoelectron spectroscopy (XPS). The GPC and FTIR spectra suggested successful bonding of MonoSilanolIsobutyl-POSS and PMS. TGA, DTG and XPS analysis revealed that MonoSilanoIsobutyl-POSS reinforced PMS are thermally more stable than the original PMS, primarily by the elimination of SiOH effects, the nanoreinforcement effect of POSS, the retardation of polymer chain motion and the formation of SiO2 protective layer.