<p>Generally, it is said that the polymer materials degradation is caused via a radical species by factors such as heat and light, oxygen, the mechanical stress. However, it has been very rare that the reaction intermediates were observed under a real condition so far. The indicated reaction schemes were actually determined inductively from the results of the product analysis after the reaction.</p><p>In this study, the thermo-mechanical degradation of the saturated hydrocarbon polyurethane was investigated using a Spin-Trapping Method, which prolongs the life time of the radical species long enough to be measured by common electron spin resonance （ESR） measurement. If the spin adduct by spin-trapping was detected by ESR, the molecular structure of the original radical can be assigned from the ESR spectrum of spin adduct. As a result, the milled polyurethane did not show any kinds of radical signals by ESR. It was thought that the enough stress was not applied on the main chain due to its low molecular weight. On the other hand, on heating between 140 ℃ and 180 ℃， five spin adducts were successfully observed by ESR. Those spin adducts were derived from ･OCH2- and ･CONH- radicals produced by dissociation of urethane O-CONH bonding, as well as -･CH- radical produced by dehydrogenation on the main chain CH2 and ･CH2- radical produced by β-scission on the main chain. Using spin-trapping method, we could clarify that the radical reactions occur on the thermal degradation of polyurethane, which had not been known until now, and show the usefulness of the spin-trapping method for the investigation of degradation analysis of polymer materials</p>