The radiation-induced graft polymerization of 4-vinyl pyridine to styrene–butadiene–styrene triblock copolymer (SBS) was investigated. Relations between the rate of grafting and the dose rate when SBS was irradiated in 4-vinyl pyridine–methanol solution, and between the rate of grafting and 4-vinyl pyridine concentration of 4-vinyl pyridine–methanol solution have been investigated. An experiment that had been carried out on SBS immersed in various 4-vinyl pyridine concentration of 4-vinyl pyridine–methanol solutions showed that the extent of swelling of SBS by the various 4-vinyl pyridine–methanol solutions increased with increasing 4-vinyl pyridine concentration. The largest rate was found at 20 vol % 4-vinyl pyridine–methanol solution. The rate was smaller at the volume percent of 4-vinyl pyridine higher or lower than 20 vol %. On the assumption that the theory of homogeneous homopolymerization could be applied to this grafting reaction, the value of kp2/kt was obtained, where kp and kt are the propagation and termination constant, respectively. The value of kp2/kt greatly decreased with increasing adsorbed concentration of vinyl pyridine–methanol solution. This decrease of kp2/kt was explained by the fact that 4-vinyl pyridine and methanol absorbed in SBS acted as a plasticizer which increased the molecular motion of the polymer. The solvent effect on the graft polymerization was also investigated. The result was explained by solubility parameter. When the chosen solvent had better solubility with the polymer, the degree of grafting was smaller. That was connected with the extent of the polymer chain mobility.