A thermoplastic resin composition having a good heat cycle property which comprises (A) a graft polymer comprising a conjugated diene rubber and an aromatic vinyl compound and a vinylic cyanide grafted thereon in a weight proportion of 5-80:15-140, the weight ratio of the aromatic vinyl compound and the vinylic cyanide being 10-90:5-50, (B) a copolymer comprising an aromatic vinyl compound and a vinylic cyanide in a weight ratio of 40:60 to 90:10 and (C) an organo-silicon compound, the weight proportion of the graft polymer (A) and the copolymer (B) being from 5:95 to 95:5 and the amount of the organo-silicon compound (C) being from 0.05 to 3.0 parts by weight to 100 parts by weight of the combined amount of graft polymer (A) and the copolymer (B).