Strain-induced crystallization (SIC) of poly(ethylene terephthalate) (PET) slightly above glass transition temperature (Tg) is investigated by adopting a sequential biaxial stretching mode in this work. Small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) are employed to study variation of orientation parameter and crystallinity of stretch films in machine and transverse draw ratio space. Our results reveal that improving chain mobility by stretching is necessary before crystallization occurs slightly above T-g, during which the total external work of sequential biaxial stretching is not dissipated but "stored" in polymer chains with high mobility. The total crystallinity is precisely correlated with the total external work, which does not relate to stretch direction and demonstrate that enhanced chain mobility have no directionality. Considering the structures that restrict the polymer chains in the initial sample, a molecular model is proposed to interpret the structure changes during SIC of PET above Tg. (C) 2016 Elsevier Ltd. All rights reserved.