This study has been designed to investigate the influence of drawing and heating conditions affecting on the change of fine structures by x-ray and birefringence measurements (in particular, on the degree of orientation in crystalline and amorphous phases) and on the mechanical properties of PET fibers. In a crystalline region, relationship between birefringence contribution, ncx, and breaking stress is expressed by a linear function, and clear structural discontinuities are observed between ncx and yield stress. In an amorphous area, relationship between birefringence contribution, na(1-x), and yield stress shows a linearity and that between na(1-x) and breaking stress is indicated by a quadratic curve. The existence of structural transformational region could easily be presumed by the fact that an inflection and tensile characteristics exists at the range of draw ratio 2.0~2.5. Particularly, it is found that an inflection phenomenon of yield stress is under the contol of a crystalline region and that of breaking stress is swayed by an amorphous area.