Tracking the Raman scattering bands of polyethylene terephthalate (PET) during thermal heating has allowed to establish criteria for monitoring the crystallinity ratio within a few seconds. As the existing criteria present a non-negligible difference from the crystallinity ratio given by differential scanning calorimetry (DSC), we propose a new way for the effective calculation of crystallinity ratio based on the evolution of the Raman bands. For the same, the band at 1727 cm-1, which is characteristic of the C=O carbonyl bonds of the ester group of PET is used. This novel approach also provides access to information on the various conformational states of the bonds under study, i.e. their proportions and positions with respect to the plane of the macromolecule aromatic rings. Evidently, this allows to reveal the characteristic thermal behavior of the PET microstructure such as the glass transition, crystallization and melting. This method can be useful for the close detection, quantification, understanding and tracing of the micromechanisms involved at the macromolecular conformations scale during thermal cycles.