Effects of particle size of Al(OH)(3) (ATH) filler on electrical properties of ethylene propylene diene terpolymer (EPDM)/ATH compounds were studied. It was found that tracking and erosion resistance of EPDM/ATH compounds decreased while dielectric properties and 90 degrees C water resistance were improved with the increase of particle size. Homocharge accumulates in the compounds, which increases first and then decreases with the increase of particle size. This was explained by the change of particle-to-particle distance due to filler size.