The thermal and rheological properties of poly(ethylene-co-trimethylene terephthalate) (PETT) copolymer are investigated. The thermal behavior of PETT copolymers is dependent on the composition. The PETT-15 and PETT-85 copolymers can crystallize, whereas the PETT-30 copolymer cannot crystallize at 5 degrees C/min cooling rate. The copolymers have a good thermal stability, even though the addition of poly(trimethylene terephthalate) (PTT) chain causes a disadvantage to the thermal stability of the copolymers. Moreover, the PETT copolymers are a typical pseudoplastic fluid exhibiting shear thinning. With increasing the shear rate or the content of PTT units, the flow activation energy decreases and the sensitivity of the shear viscosity to the melt temperature declines. The PETT copolymer filaments have intermediate elastic recovery and dyeability between poly(ethylene terephthalate) (PET) and PTT filaments. With increasing the PTT content, the elastic recovery and dyeability of PETT copolymer filaments increase. That is to say, introducing PTT units as a minor component into the macromolecular chains is an available means to improve the properties of PET filament. The obtained PETT copolymer filaments blend the advantage of the mechanical property of PET and the elastic and dyeability of PTT filament together into one polymer and possess a softer feeling and a higher extension. POLYM. ENG. SCI., 50:1689-1695, 2010. (C) 2010 Society of Plastics Engineers