Vinyl trimethoxysilane and vinyl triethoxysilane grafting reactions, induced by dicumyl peroxide, of LDPE, HDPE, and LLDPE were investigated. The apparent activation energy of vinyl trimethoxysilane grafting reactions was positive when the reactions were induced by 0.2 phr of the peroxide. The apparent activation energies were negative when 0.05, 0.1, 0.15, and 0.25 phr of peroxide were used. The extents of vinyl trimethoxysilane grafting reactions of polyethylenes were in the order of LLDPE ΟΎ LDPE ΟΎ HDPE, although the extents of peroxide cross-linking were in the order of LDPE ΟΎ LLDPE ΟΎ HDPE. As compared with vinyl trimethoxysilane, vinyl triethoxysilane produced a relatively high extent of grafting reactions of LDPE but showed a relatively low rate of water cross-linking reactions of the silane-grafted LDPE. The investigation of effects of the amount of peroxide on the vinyl trimethoxysilane grafting reaction heats demonstrated that the extent of silane grafting reactions increased proportionally as peroxide was increased until a certain amount (the value was dependent on the amount of silane used). Beyond this amount of peroxide, the silane grafting did not increase, whereas the peroxide cross-linking appeared to increase significantly with increasing amounts of peroxide.