We studied the chemical reaction process of polypropylene (PP), ethylenepropylene copolymer (EPM), and ethylene-propylene-diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120°C and the behavior and kinetics of the reaction process were observed at 180°C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction.