Ethylene propylene diene terpolymer (EPDM) rubber nanocomposites have been prepared by incorporating 10 phr of organically modified montmorillonite (OMMT) nanoclay and various percentages of fumed silica (10, 20, and 30 phr) onto EPDM rubber composite using two roll mill and crosslinked by gamma radiation. The effect of radiation dose on the mechanical properties namely tensile strength and elongation at break and the physical properties namely solubility and swelling were investigated. Thermogravimetric analysis was carried out to analyses the thermal stability of the nanocomposites. Volume resistivity of the nanocomposites was also measured. The results showed that the mechanical, volume resistivity, and thermal properties of the irradiated EPDM rubber containing OMMT were increased by adding of the fumed silica. The effect of N,N-m-phenylenedimaleimide coagent on the mentioned properties of the EPDM rubber nanocomposites was studied. The results showed that the physical, mechanical, and thermal properties of the irradiated EPDM rubber nanocomposites improved with the addition of the coagent due to the increase of the crosslinking density. The inclusion of both the 30 phr fumed silica and the coagent in the nanocomposite irradiated to 50 kGy leads to a synergistic effect. J. VINYL ADDIT.