The morphology and dynamic mechanical properties of the composite materials [P(St–DVB)/PVC systems], prepared by copolymerization of styrene and divinylbenzene in the presence of fine PVC powder, were studied by electron microscopy and the dynamic mechanical test. The composite material (PSt/PVC composite), prepared by the polymerization of styrene in the presence of fine PVC powder, contains grafting polystyrene (PSt) onto PVC, which improves the compatibility of PSt and PVC. This result also suggests the formation of the graft copolymer of styrene–divinylbenzene onto PVC in the P(St-DVB)/PVC systems. Electron microscopy and the dynamic mechanical test indicate that P(St-DVB)/PVC systems have the two-phase nature with a styrene–divinylbenzene copolymer as the continuous phase [P(St-DVB) phase] and a PSt/PVC composite as the dispersed phase (PSt-PVC phase), in which PSt penetrates into the PVC domain. The domain size of the dispersed phase is 0.5–2μ. The crosslinking density of the P(St-DVB) continuous phase is larger than that estimated from the recipe. One of the reasons for this is ascribed to the difference of the diffusion constants of styrene and divinylbenzene into the PVC particles on the paste formation and polymerization process.