The process of semicrystalline polymers thermo-oxidative degradation was analyzed on the example of isotactic polypropylene (iPP). Oxygen degradation takes place mainly in the amorphous phase into which oxygen diffuses. This leads to oxidation (formation of hydroperoxides) and next the chains scission. The polymer becomes brittle and breaks down into small pieces (e.g. microplastics). The crystalline phase is more difficult to access for oxygen that initiates the thermo-oxidation process. An increase in the crystallinity degree and a decrease in the viscosity of the solutions diluted in p-xylene as a function of the degradation time were observed. The number of iPP chain scission (change in molecular weight) as a function of degradation time was also calculated. These values depend on the polymer initial crystallinity degree and its supermolecular structure.