PURPOSE: Provided are a rubber modified thermoplastic resin composition which is excellent in weather resistance, impact strength, tensile strength, fatigue strength, heat resistance and appearance, and its preparation method. CONSTITUTION: The rubber modified thermoplastic resin composition comprises 2-15 parts by weight of an ethylene--olefin-based copolymer; 75-85 parts by weight of an aromatic vinyl compound and a cyano vinyl compound; and 5-30 parts by weight of a graft copolymer of an ethylene--olefin-based copolymer, an aromatic vinyl compound and a cyano vinyl compound. Preferably the -olefin of the ethylene--olefin-based copolymer has three to eight carbon atoms and is selected from the group consisting of propylene, 1-butene, 1-pentene, 1-hexene and 1-octene; the ethylene--olefin-based copolymer has a Mooney viscosity (ML1+4, 100 deg.C) of 25-45 and a weight average molecular weight of 1x10¬5 to 4 x10¬5; and the ratio of the aromatic vinyl compound to the cyano vinyl compound is 70-80 : 20-80 by weight.
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