The effect of an antioxidant on the reaction mechanisms of chemical crosslinking of polyethylene (PE) with dicumyl peroxide (DCP) at high temperatures was investigated using electron spin resonance (ESR). For sulfur- and phosphorous-type antioxidants, changes of radical species and their contents during the PE crosslinking reaction were observed. It was confirmed that these antioxidants reacted preferentially with radicals yielded by decomposed DCP, restraining the crosslinking of PE by the increased antioxidant content. The compound of DCP and antioxidant decomposed to form 2-phenyl isopropyl radicals.