To obtain a modified polyethylene terephthalate capable of being molded into a blow molded bottle suitable for use for bottling a liquid such as beverage, etc., in a high productivity and to provide a method for producing the same.

This modified polyethylene terephthalate is obtained by introducing (A) a polyethylene terephthalate obtained by subjecting an esterified substance of terephthalic acid and ethylene glycol to condensation polymerization reaction in a molten state and (B) a crystalline thermoplastic resin selected from a polyolefin, a polyamide and an ethylene/unsaturated carboxylic acid copolymer both in a molten state into a mixer, mixing and stirring the polyethylene terephthalate and the resin to give a crystalline thermoplastic resin-containing polyethylene terephthalate, granulating the crystalline thermoplastic resin-containing polyethylene terephthalate into a chip state and, if necessary, heating the obtained chip-like crystalline thermoplastic resin-containing polyethylene terephthalate to a temperature lower than the melting point of the polyethylene terephthalate forming the crystalline thermoplastic resin- containing polyethylene terephthalate so as to raise the intrinsic viscosity of the polyethylene terephthalate.