The compatibilizing effect of poly(styrene-co-acrylonitrile) (SAN) whose acrylonitrile (AN) content is 25 wt % (SAN 25) in poly(acrylonitrile-co-butadiene-costyrene) (ABS)/poly(vinyl chloride) (PVC) blend was studied when the AN content of the matrix SAN in ABS was 35 wt % (SAN 35). When some amount of matrix SAN 35 was replaced by SAN 25 in a ABS/PVC (50/50 by weight) blend, the mixed phase of SAN and PVC at the interface was thickened, and about a twofold increase of impact strength was observed. The changes in morphology, dynamic mechanical properties, and rheological properties by the compatibilizing effect of SAN 25 were observed.