In this paper, EPDM/mica composites were prepared by filling synthesized mica and natural mica separately into ethylene-propylene diene terpolymer (EPDM) using melt blending technique. Microstructures, electrical properties, gas resistance, and mechanical properties of two EPDM/mica composites were investigated systematically. FTIR show that hydroxyl groups exist on the surface of the micas. These structural hydroxyls could be active sites conducive to the surface modification of mica. XRD analyses reveal that the natural mica is crystalline and the synthesized mica is amorphous. After being modified with silane coupling agent Si69, mica was only exfoliated into smaller micron agglomerates dispersing in EPDM, but the dispersion of amorphous synthesized mica was better. So the EPDM/synthesized mica composite possessed better mechanical property, electrical insulation property, and gas permeability resistance. It is expected that better improvement would be achieved, if mica is exfoliated further into nanosheets dispersing in the rubber matrix. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 3184-3192, 2010