Abstract: Methyl methacrylate-butadiene-styrene (MBS) core-shell copolymer was synthesized by grafting methyl methacrylate and styrene on SBR rubber particles using the seed emulsion polymerization. MBS/PVC blends were obtained by melt blending. The structure characteristics of MBS were analyzed, and the effects of different core-shell ratios in MBS on the disperse morphology, mechanical property and transparency property of MBS/PVC blends were investigated for obtaining high ductility and transparency MBS/PVC blends. With increasing the shell amount of MBS, dispersion states showed aggregation, network dispersion and homogeneous dispersion, the SBR brittle-ductile transition content for MBS/PVC blends decreases and the transparency of MBS/PVC blends decreases. The toughening mechanism of PVC blends with small MBS particles is shear yielding of the PVC matrix.