Many useful physical and chemical properties of polymer materials can be damaged in the presence of radiation such as UV light, γ-irradiation and electron beams. To seek a good way to solve these problems, clay was introduced into the tri-block copolymer styrene–butadiene–styrene (SBS) matrix to form SBS/clay nanocomposites. The formation of intercalated SBS/clay nanocomposites is due to polystyrene (PS) blocks intercalating into the clay layers as characterized by X-ray diffraction (XRD). When the nanocomposites are exposed to γ-radiation under atmospheric oxygen, the decrease of their storage modulus relative to SBS can be efficiently restrained by the clay at temperatures above the glass transition temperature of the polybutadiene (PB) block. The layers of organophilic clay can not only absorb a large amount of radiation to protect the chains of SBS from being irradiated, but also produce a large number of free radicals to recombine the chain free radicals of SBS, so that the broken chains of SBS can be grafted onto the surface of the layers of the organophilic clay, as confirmed by electron spin resonance (ESR) and gel fraction measurements. Therefore, the formation of SBS/clay nanocomposites can remarkably improve the radiation stability of SBS.