A thermally stimulated current (TSC) was used to study the relaxation of polystyrene ( PS), the ethylene-propylene diene monomer (EPDM), and the grafted copolymer of EPDM with styrene (EPDM-g-St). The effect of the measuring conditions on the relaxation of PS is discussed. Some relaxation parameters of PS, EPDM, and EPDM-g-St were calculated in two different ways. In addition, the TSC spectra of PS/EPDM and PS/EPDM-g-St showed that PS/EPDM was an immiscible system, but there existed a special interaction between the plastic phase and the rubber one in PS/EPDM-g-St blends. The compatibility of PS blends was evaluated in terms of their compositions. The results of inverse gas chromatography(IGC) agreed well with those of the TSC measurements.