A rubber modified vinyl-based graft copolymer of the present invention has a core-shell structure in which a shell is formed by graft-polymerizing a monomer mixture to a core comprising a rubbery polymer, wherein the monomer mixture comprises: an aromatic monomer comprising an α-substituted aromatic vinyl-based monomer represented by chemical formula 1 and an aromatic vinyl-based monomer excluding the α-substituted aromatic vinyl-based monomer; a cyanovinyl-based monomer; and a maleimide-based monomer. A thermoplastic resin composition containing the rubber modified vinyl-based graft copolymer has excellent impact resistance, heat resistance, appearance characteristics and the like.