Polyethylene Terephthalate(PET) from ethylene glycol and terephthalic acid or dimethyl terephthalate usually contains small amounts of diethylene glycol terephthalate(DEGT) units due to side reactions in polymerization. DEGT units do not participate in the formation of crystalline structure, and only very small amounts are incorporated in crystal as lattice defects. Most DEGT remain in amorphous regions. DEGT can reduce T$_{g}$ because of increase in main chain flexibility and the crystallizable segment length, resulting in an increase in the amount of amorphous regions of the PET fiber. These effects of DEGT can be anticipated to exhibit a great role in the improved dyeability of PET fiber without addition of any kinds of monomers or modifiers. Literature survey indicated that previous studies were mainly focused on the effects of DEGT on crystallization behavior and thermal stability of bulk PET. Studies on the effect of DEGT on PET filaments fiber could not be found in the literatures. In addition, no study is available to elucidate the effects of DEGT on the dyeability of PET. Therefore, to improve the dyeability of PET fiber the present study investigated the effects of DEGT contents on the structural properties and dyeability of PET filament. Polymer chips synthesized with the addition of diethylene glycol(DEG) in the polymerization were converted to the filament fiber by subsequent steps of spinning and drawing. Effects of various DEGT contents on the crystallinity of PET fiber, crystal size. and orientation factors of crystal and amorphous regions, and consequently, structure of amorphous regions which is an important factor in determining dyeability of the fiber were studied. Through evaluation of T$_{g}$ and dynamic mechanical properties, the effects of DEGT contents on the amorphous structure of PET fibers were also studied. Dyeing experiment was further carried out to determine the effect of DEGT contents on the uptake and diffusion coefficient of dye in the PET fiber.ber.