In order to investigate the role of Carrier in the disperse dyeing of the oriented and crystalline PET fiber in connection with the dyeing properties and the fine structure of fiber, the changes of tan$delta$-temperature curve, crystallinity and dyeing properties caused by the carrier treatment were measured. The results of experiments were as follows; with the increase of carrier concentration in the range of 0∼6g/ι, (1) the height of ${alpha}$-peak of tan$delta$-temperature curve was increased considerably by the carrier treatment and T${alpha}$ was not changed, but the tan$delta$-temperature curve was broadened at lower temperature. (2) degree of crystallinity was increased (3) the diffusion coefficients and saturation values were showed a direct proportion with the changes of ${alpha}$-peak width (half width) By the carrier treatment, it was considered that the-free volume in amorphous region of PET fiber was increased with the decrease of crystal orientation and the loose packing of amorphous chains was increased with the increase of crystallinity.