AbstractWith high‐density polyethylene (HDPE) and low‐density polyethylene (LDPE) films the grafting reactions were performed by the preirradiation method. By holding the total absorption dose constant irradiation time was varied. The initial rate of grafting decreased with irradiation time. The relative concentration of alkyl radical in the polyethylene film also decreased with irradiation time, but the relative concentration of allyl increased. The differences in the ESR spectrum before and after the introduction of styrene indicate that the allyl‐type radical reacted with styrene. To elucidate these results the allyl radical in the amorphous region was consi