Abstract: A rubber-modified thermoplastic resin is obtained by polymerizing (b) an aromatic vinyl compound and (c) a vinyl cyanide compound in the presence of (a) a rubbery polymer, and in which the content of the component (a) is 50 to 85% by weight, the content of the component (b) is 5 to 48% by weight and the content of the component (c) is 2 to 45% by weight, and which resin has a Q value of 30.times.10.sup.-4 to 50.times.10.sup.-3 and a grafting degree of 5% by weight or more. A thermoplastic resin composition comprising the above rubber-modified thermoplastic resin and at least one other thermoplastic resin is prepared.

Abstract: A thermoplastic resin having an excellent thermal stability and a high heat resistance is prepared by initiating polymerization of 65-85 parts by weight of a monomer mixture (A) consisting of 50-85% by weight of .alpha.-methylstyrene, 3-20% by weight of methyl methacrylate, 2-20% by weight of acrylonitrile, and 0-25% by weight of an aromatic alkenyl compound other than .alpha.-methylstyrene, continuously or intermittently adding, on the way of the polymerization, 2-15 parts by weight of a monomer (B) consisting of acrylonitrile or a mixture thereof with an aromatic alkenyl compound other than .alpha.-methylstyrene to the polymerization system while continuing the polymerization, further adding to the polymerization system 5-25 parts by weight of a monomer (C) consisting of an aromatic alkenyl compound other than .alpha.