The thermoplastic resin composition of the invention comprises (A) from 50 to 90 parts by weight of a copolymer of an aromatic vinyl monomer and an unsaturated dicarboxylic acid anhydride, e.g. copolymer of styrene and maleic anhydride, (B) from 50 to 10 parts by weight of an ABS or MBS resin and (C) from 0.01 to 1.0 part by weight of an organopolysiloxane. In contrast to conventional polymer blend compositions composed of the above mentioned components (A) and (B), the inventive resin composition is capable of giving a shaped article having a greatly improved impact strength without adverse influences on the heat resistance and surface luster of the shaped articles molded of a resin composition of this type inherent thereto.