A MBS resin useful as a reinforcement for vinyl chloride polymers and capable of improving the impact resistance of the vinyl chloride polymers without impairing the transparency thereof, which is prepared by graft-polymerizing a monomer mixture comprising an aromatic vinyl compound and an alkyl methacrylate onto a butadiene copolymer comprising styrene and butadiene, the butadiene copolymer being present in the polymerization system in a higher proportion as compared with a conventional MBS resin and present in the form of an aqueous latex of particles having a structure such that a portion formed in the last stage of a polymerization for the preparation of the butadiene copolymer amounting to 20% by weight of the produced butadiene copolymer, has a very high butadiene content of 80 to 100% by weight and moreover has at least 5% by weight higher butadiene content than the average butadiene content of the produced butadiene copolymer.