||POSS disulfide was synthesized from the oxidation of POSS thiols and it was incorporated into sulfur cured EPDM to form EPDM/POSS nanocomposite. Curing behavior of EPDM/POSS disulfide compounds were examined by oscillating disk rheometer(ODR), and morphology, thermal and mechanical properties of the nanocomposite vulcanizates were investigated by WAXD, TEM, DMTA, tensile testing and TGA. It was found that the POSS disulfide participate in sulfur vulcanization of rubber and improve its crosslinking efficiency. Morphological analysis revealed that POSS nanocrystals were dispersed uniformly on the nanoscale throughout the EPDM matrix. There were significant increase in mechanical properties and thermal stability of sulfur vulcanized EPDM/POSS nanocomposites with a small amount of the POSS loadings.