Polymorphic behaviors of two types of syndiotactic graft copolymers (syndiotactic polystyrene (SPS) having side chains of polyisoprene (PIP), namely SPS-graft-PIP, and SPS having side chains of atactic polystyrene (APS), namely SPS-graft-APS) were investigated. Though the as-prepared samples of the both types were in the delta form as well as the homopolymer, they showed different polymorphic behaviors. Furthermore, while SPS-graft-PIP and SPS-graft-APS with the lower contents of the APS side chains could crystallize from the melt, SPS-graft-APS with the higher contents of APS was in an amorphous state. The effect of the graft chains on crystallinity was discussed in terms of the friction force for reptation and the cohesive power; the former may be increased by the side chains in both types of the graft copolymers, while the latter may be enhanced only in the case of SPS-graft-PIP, compensating the increment of the friction force.