Four organoclay (OC)/ethylene-propylenediene rubber (EPDM) nanocomposites with different ethylene contents were prepared by melt blending. X-ray diffraction spectrum (XRD) and transmission electronic microscope (TEM) photos showed that OC/EPDM nanocomposites were intercalated, and the ethylene content had little influence on the dispersion of OC. The addition of OC prolonged the optimum cure time and reduced the crosslink density of OC/EPDM. The improvement in tensile strength of OC/EPDM nanocomposites with high ethylene contents (67-70%) was larger than that of OC/EPDM nanocomposites with low ethylene contents (52-52.5%). XRD results of the stretched samples testified that the extension promoted orientation of silicate layers, and induced crystallization of polyethylene (PE) segments in OC/EPDM nanocomposites with high ethylene contents. The highly oriented microfibrillar structure and more oriented amorphous chains, which resulted from strain-induced crystallization of PE segments and the orientation of clay layers in OC/EPDM nanocomposites with high ethylene contents (67-70%), should be responsible for larger improvement in tensile strength than that of those nanocomposites with low ethylene contents (52-52.5%). (c) 2005 Wiley Periodicals, Inc.