The invention relates to an acrylic impact modifier having a core-shell structure that provides an acrylic impact modifier composition comprising (a) a rubber core containing an alkyl acrylate polymer comprising at least two layers having different cross-linking densities, and (b) a shell containing an alkyl methacrylate polymer; to a process for the preparation of the acrylic impact modifier; and to a poly(vinyl chloride) composition comprising it. The acrylic impact modifier imparting excellent impact resistance was invented by employing multi-stage polymerization and at the same time by controlling the swelling index of rubber particles by changing the degree of cross-linking from stage to stage. And the poly(vinyl chloride) comprising the impact modifier of the present invention has good weatherability as well as excellent impact strength.