An in situ study of structural development in amorphous poly(ethylene terephthalate) under uniaxial deformation above Tg (90°C) was carried out using synchrotron WAXD. The results showed several new insights into this subject. The load-strain curve can be divided into three zones (I, II, III) with each zone having a unique structure and property relationship. Zone I represents the development of mesophase. Zone II represents the strain-hardening region, which is marked by the development of a three-dimensional network of imperfect crystallites and mesophoric chains. Zone III represents the further formation of mesophase and the growth of a stable crystal phase, which is characteristic of a constant load-strain ratio.