Isothermal crystallization of a high molecular weight PEO (Mw= 4.000.000) has been investigated using photoDSC. Combining light irradiation, heating and DSC analysis, photoDSC demonstrates a good capability to follow the in situ photo and thermo aging of semi crystalline polymers. Isothermal crystallization of PEO has been performed at 55°C. After aging at different temperatures ranging from 0 to 90°C and for various periods of time, the kinetics of this crystallization has been found fitting Avrami theory. Avrami exponent, n, was found between 0.9 and 1.4 revealing a one-dimensional growth process. It was also found that isothermal crystallization rate (i.e. reciprocal crystallization half-time) was time exposure, aging temperature and light intensity dependent which makes this kinetics parameter a good indicator to follow and to compare the degradation of the semi-crystalline polymers.