POSS/PMMA composite was synthesized by atom transfer radical polymerization (ATRP) at 110 A degrees C using commercial POSSCl as an initiator and CuCl/2,2'-bipyridine as catalyst system. The structures of POSS/PMMA and POSSCl were characterized by Fourier transfer infrared spectroscopy, Nuclear magnetic resonance spectroscopy, Ger permeation chromatography, X-ray diffraction and X-ray photoelectron spectroscopy, which confirmed that Si-Cl bond on POSS cage could successfully initiate the ATRP of methyl methacrylate, so there is only one POSS unit in a PMMA chain. The thermal properties of POSS/PMMA were investigated by Differential scanning calorimetry and Thermogravimetric analysis, the results show that the incorporation of POSS cage results in the enhancement of the glass transition temperature and the decomposition temperature of PMMA, which is mainly attributed to the mono-dispersion of POSS in PMMA matrix at molecular lever.