Poly(ethylene terephthalate)(PET) was chemically modified through copolymerization of bis(2-hydroxyethyl) terephthalate (BHET) and 2,2-bis(4-(2-hydroxyethoxy) phenyl) propane (BHPP) as a flexible chain moiety to enhance high-speed spinnability by retarding the stress-induced crystallization during spinning. The properties of the copolyester were investigated using GPC, DSC(differential scanning calorimetry), TGA(thermogravimetry analysis), and proton-NMR. From the result of GPC, it was found that the copolyesters had higher molecular weight and narrower molecular weight distribution than those of PET obtained from BHET only. Non-isothermal crystallization measurement showed that $T_{cm}$ (melt-crystallization temperature) decreased and $T_{cc}$ (cold crystallization temperature) increased with increasing amount of comonomer. It seems that the rate of crystallization of copolymer might be retarded during melt-spinning. Isothermal melt-crystallization measurement suggested that the rate of crystallization was lowered and Avrami index decreased with increasing amount of comonomer. However, incorporation of the comonomer did not affect the crystal structure of PET.