The radiation graft copolymer of styrene onto MgO has been characterized by FT-IR, X - ray diffaction, XPS and GPC. It is shown that the graft copolymer has some peaks of MgO and polystyrene in IR and X - ray diffraction spectra respectively. It was found that there were some at 285.0, 530.0 and 87.6 eV in the spectra of the blend, which blongs to C,1S, O,1S, and Mg,2S, respectively, while the peak position of C,1S, O,1S, and Mg,2S, of the MgO - g-styrene copolymer were 284.0, 531.7 and 88.4 eV respectively. This shows that the active centers caused by the defect of Mg atom after irradiation react with styrene to form monomer free radical, then grafting reaction occurs by the free radical mechanism. The ratio of oxygen to carbon (O/C) of graft products decreases with the increasing of the grafting yield. The molecular weight of graft copolymer is higher than that of homo-polymer, but the molecular weight distribution is broader in the graft copolymer.