In order to clarify the effect of thermal degradation on the structure of polypropylene materials, we investigated the changes in molecular weight distribution. The samples of polypropylene were degraded iso-thermally at 190 A degrees C at different time intervals. The molecular weight distribution was significantly changed with thermo-degradation time, and the carbonyl index increased drastically for 40 min degraded samples, where the molecular weight distribution started splitting into two peaks. The results imply that heterogeneous degradation proceeded in this system. Sequentially, the weight distribution of the oligomeric products observed was discussed on the basis of chain scissions; these results indicate that there are some kinetically favored scissions occurring near the oxygen-centered radicals.