Thermoplastic polyester resin composition

A thermoplastic polyester resin composition comprising a thermoplastic polyester resin (A) and a graft modified mixture of ethylene.1-butene copolymers (B) in a weight ratio of 95/5 to 50/50, wherein

• (1) the graft modified mixture (B) is essentially composed of
• (i) an ethylene.1-butene random copolymer (C) which contains units derived from 1-butene in an amount of 5 to 15 mol % and has an intrinsic viscosity [.eta.], measured at 135.degree. C. in a decalin solvent, of 0.5 to 3.5 dl/g, a glass transition temperature (Tg), measured by a DSC method, of -50.degree. C. or less and a crystallinity, measured by an X-ray diffraction method, of less than 40%,
• (ii) an ethylene.1-butene random copolymer (D) which contains units derived from 1-butene in an amount of 16 to 25 mol % and has an intrinsic viscosity [.eta.], measured at 135.degree. C. in a decalin solvent, of 0.5 to 3.5 dl/g, a glass transition temperature (Tg), measured by a DSC method, of -50.degree. C. or less and a crystallinity, measured by an X-ray diffraction method, of less than 15%, and
• (iii) graft units derived from at least one monomer selected from unsaturated carboxylic acids and derivatives thereof and bonded to at least one of the two copolymers, and
• (iv) the graft modified mixture (B) contains units derived from 1-butene in an amount of 11 to 19 mol % based on the total of units derived from ethylene and 1-butene and has an intrinsic viscosity [.eta.], measured by the above method, of 0.5 to 3.5 dl/g, a glass transition temperature, measured by the above method, of -50.degree. C. or less, and a crystallinity, measured by the above method, of less than 10%,
• (2) the amount of the above graft units is 0.01 to 10% by weight based on the total amount of the copolymer (C) and the copolymer (D), and
• (3) the weight ratio ((C)/(D)) of the copolymer (C) to the copolymer (D) is 10/90 to 95/5.