The mechanical and electrical properties of natural rubber (NR)-alumina (AI2O3) composites crosslinked with dicumyl peroxide (DCP) were investigated. Scanning electron micrographs indicate the interaction of the C-C microstructure and that alumina particles are moderately dispersed in the matrix. The X-ray diffraction patterns indicate that Al2O3 is of the beta phase polytype that possesses high innic conductivity. The electrical conductivity of the composites with DCP is greater by nearly three orders of magnitude than that of NR-Al2O3 composites without DCP and pure NR. The increase in the electrical conductivity and the mechanical property is caused by the strong C-C bond, the tunnelling phenomenon and the ionic polarisation of the alumina particles acting as the dispersed phase in the elastomer matrix.