Deformation-induced phase transitions and superstructure formation in poly(ethylene terephthalate) (PET) were studied by means of in-situ synchrotron small-angle X-ray scattering (SAX-S) and wide-angle X-ray diffraction (WAXD) as well as Raman spectroscopy. The deformation conditions involved uniaxial stretching of quenched PET films at a temperature just, below its glass transition temperature (T,), where a notable "plastic deformations stage was ob-served. WAXD results indicated that the initial sample contained a "slush" structure (amorphous + nematic)., whereby, deformation induced oriented amorphous, nematic, smectic (C and quasi-A), and stable triclinic crystalline phases. SAKS results indicated that the fibrillar superstructure was formed upon the formation of oriented slush. In-Situ Raman spectroscopic data revealed the orientation information on ethylene glycol and benzene ring as well a -: the gauche-trans transition in deformation of PET chains, which are in good agreement with X-ray results. A mechanism for deformation-induced phase transitions and for hierarchical structure formation has: been proposed to correlate the structural information with the mechanical properties.