Abstract: Carbon nanotube/ethylene-propylene rubber composites were prepared by gas-phase polymerization of the monomers in an autoclave loaded with carbon nanotubes under agitation. The aggregation of rubber particles during polymerization was effectively prevented by adding more than 6.76 wt% carbon nanotubes that were well dispersed in the composites. The addition of carbon nanotubes had no effect on the melting and thermal decomposition temperatures of the rubber. The composites with carbon nanotubes had a significantly better conductivity and tensile strength than those with conductive carbon black or those obtained with the same formulations using a mechanical blending method.