The period of conversion from alkyl radicals to polyengl radicals in gamma-irradiated isotactic polypropylene (i.e. PP) is ≲ 6 days; while in 6.8%APP and 7.6% EPP (APP and EPP are PP containig hexadecane and hexadecene-1, respectively). It was found to be more than 10 days. It was also measured that the initial gelation dose of PP, 6.8% APP, 4.0% EPP and 7.6%EPP are 12.6, 20.7, 30.5 and > 108 Mrad, respectively. This indicates that both of the additives inhibit the radiation crosslinking of PP, and hexadecene-1 shows a stronger effect than hexadecane. It was assumed that hexadecene-1 offers its active allyl H atoms to reduce the radical yield of PP, and then inhibit the radiation crosslinking of PP more strongly than hexadecane. However, the whole mechanism related to the effect of the additives on PP radical connversion needs further research.