Functionalization of graphene oxide (GO) with octavinyl polyhedral oligomeric silsesquioxane (OvPOSS) is achieved by gamma-ray radiation-induced grafting under mild conditions. The grafting yield of OvPOSS-grafted GO (GOP), calculated by the content of silicon in GOP using X-ray photoelectron spectroscopy, increases gradually with the increase in the absorbed dose. The grafting of OvPOSS onto GO not only improves its thermal stability, but also increases its distance between the adjacent graphitic layers which could improve the reactivity of GO. In contrary to the hydrophobic GOP prepared via the traditional chemical method, it dramatically becomes more hydrophilic with the increase in the absorbed dose due to the introduction of nitrogen-containing group under irradiation, and the surface of GOP becomes super-hydrophilic at an absorbed dose of 50 kGy. Furthermore, GOP still has excellent electrical insulation property due to the grafting of insulated OvPOSS. Consequently, this work paves a feasible and green way for the functionalization of GO with improved performances.