Poly(ethyleneterephthalate)(PET) was chemically modified through copolymerization of bis(2-hydroxyethyl) terephthalate (BHET) and neopentyl glycol (NPG). NPG was used as flexible chain moiety to enhance spinnability by retarding the stress-induced crystallization during spinning. The properties of the copolyester were investigated by using GPC, DSC(differential scanning calorimetry), TGA(thermogravimetry analysis), and $^1H-NMR$. From the result of GPC, it was found that the copolyesters were able to get higher molecular weight and narrower molecular weight distribution than those of PET obtained from BHET alone. Non-isothermal crystallization behavior of the copolymers was investigated by differential scanning calorimetry (DSC). Non-isothermal crystallization measurements showed that, with increasing the amount of comonomer, Tcm decreased.. It seems that the rate of crystallization of copolymer might be retarded during melt-spinning. Isothermal melt-crystallization measurement demonstrated that the rate of crystallization and the Avrami index were reduced with increasing the amount of comonomer.