The mechanical properties of EB-irradiated natural rubber (NR) composites were improved by a synergistic effect of adding silica (SiO2) and graphene nanoplatelets (GE) mixture into NR matrix, reported in a previous work. In this study, dynamic mechanical analysis (DMA) was performed, from which volume resistivity and degradation properties were determined. Latex NR compound filled with SiO2/GE mixture was cured by EB irradiation to form NR composites. SEM images and the EDXS elemental mapping indicated good dispersion of GE and SiO2/GE in the NR matrix. The DMA showed that the storage moduli of NR composites filled with fillers were higher than that of NR-EB. Using 2.5 phr of fillers, which is below the percolation threshold, volume resistivity properties of all NR composites remained unchanged relative to the pure NR and with varying EB doses in the range of 100–250 kGy. Finally, it was found that adding fillers into NR composites caused a decrease in water absorption of irradiated NR composite at 70 °C and a relative humidity of 80%. According to the authors’ previous work and the present results, this novel NR composites showed a potential in devices related to high-voltage application requiring electrical insulation with enhanced mechanical properties.