A real time quantitative thermo-optical characterization of polyethylene terephthalate PET fiber was performed using a modified cross-polarized optical microscope. The thermal cycles were produced by a hot-stage CSS 450 and the quantitative optical measurements were done by placing an optical module, recently developed in our research group, into the slot of the microscope´s analyzer. The module has two adjacent photocells, one of them covered by a polarizer filter. The first photocell measures changes in the transmitted light intensity resulted by the scattered light (i.e. turbidity) and the second (covered by the filter) the cross-polarized light intensity. The signals are collected via software, developed in the LabView 8.6 platform, which in real time calculate, present in the screen and save the data. In addition, it also controls all parameters set on the CSS 450. Finally, a digital camera was coupled in the ocular tube of the microscope to allow for both colored image capturing and video recording. The structural changes in the fiber arising from the thermal protocol were assessed by both optical signals, which reveal the PET thermal transitions: glass transition, cold crystallization and melting. Additionally the observation of the interference color formed during the thermal cycles, quantitatively as optical path differences, is another way of displaying the thermal transitions as a function of temperature. This helps understanding the PET molecular mobility including glass transition, cold crystallization and melting. As control the optical signals were compared with DSC scans showing good reproducibility, validating the thermo-optical method. Furthermore it has the advantage in providing additional information including level of fiber orientation and stretching.