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An in situ study of structural formation of amorphous poly(ethylene terephthalate) (PET) during uniaxial deformation above its Tg (at 90 °C) was carried out by wide-angle X-ray diffraction (WAXD) with synchrotron radiations. Results indicate that the relationships between structure and mechanical property can be divided into three zones: I, II and III. In Zone I, oriented mesophase is induced by strain, where the applied load remains about constant but the amount of mesophase increases with strain. In Zone II, crystallization is initiated from the mesophase through nucleation and growth, where the load starts to increase marking the beginning of the strain-hardening region. The initial crystallites are defective but they form an effective three-dimensional network to enhance the mechanical property. The perfection of the crystal structure and the orientation of the crystals all increase with strain in this zone. In Zone III, the ratio between load and strain is about constant, while the stable crystal growth process takes place until the breaking of the sample. The sample damage is probably dominated by the chain pull-out mechanism from the crystal amorphous interface. The increase in molecular weight was found to enhance the overall mechanical properties such as the load to induce the mesophase and the ultimate tensile strength before breakage. [Copyright &y& Elsevier]