The effect of radiation cross-linking on the properties of Engage® 8200 ethylene–octene copolymer (EOC) with multi-walled carbon nanotube (CNT) nanocomposites was evaluated. An ultra-sound assisted technique combined with thermoplastic mixing was used to make EOC/CNT composites with a wide ratio of CNT concentrations (0 to 15 wt%). Composite films were irradiated by 5 MeV accelerated electrons at relatively high doses (150 and 300 kGy), and their structure and mechanical and dielectric properties were compared. Gel fraction measurements indicated dominant cross-linking of EOC with the rise of the absorbed dose. Cross-linking as well as chain scission of macromolecules in the presence of CNTs caused a certain change in mechanical properties. Dielectric measurements indicated a decrease in ac conductivity and a change in dielectric permittivity, mainly associated with prevented charge movements between CNTs incorporated in the spatially cross-linked macromolecular structure of EOC compared to that of unirradiated EOC/CNT composites.