The thermal degradation mechanism of poly(propylene) (PP) fabric from room temperature to 220 °C was analyzed using the electron spin resonance (ESR) spin-trapping technique. This method was adopted to prolong the lifetime of the short-lived radical intermediates generated by thermal degradation with the object of inspecting their molecular structure. A spin-trapping reagent, 2,4,6-tri‑tert-butylnitrosobenzene (TTBNB), was impregnated into the PP fabric using supercritical CO2 (scCO2) treatment in the presence of a co-solvent. The impregnation efficiency and effect were evaluated using proton nuclear magnetic resonance (1H NMR) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM). ESR analysis showed that the degradation of PP is initiated from the tertiary carbon positions via two pathways: homolysis to generate ·CH3 and −CH2−·CH−CH2− radicals, and hydrogen abstraction to generate −CH2−·C(CH3)−CH2− radicals. These radicals are also attacked by oxygen to form hydroperoxides, which then decompose to form the alkoxy radicals −CH2−CH(O·)−CH2− and −CH2−C(CH3)(O·)−CH2−. These radicals are involved in the subsequent β-scission reactions that produce primary and secondary carbon radicals. Alkoxy radicals also take part in the β-scission reactions involving the main chain carbon radicals, which have a hydroxyl group in the β-position, and in hydrogen abstraction from a five-membered ring at temperatures higher than the melting-point to produce an α‑hydroxyl carbon radical, ·CH(OH)−. These inferred processes involved in the thermal degradation of PP fabric were supported by other methods of analysis, including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA).