Almnina trihydrate(ATH) plays an important role in improving the tracking resistance of EPDM, silicone rubber and epoxy compound. This study describes the influence of the median particle size of alumina trihydrate(ATH) filler on the tracking resistance. A fixed 100 phr concentration of the filler of ATH was used for all particle sizes from 0.7 to 20 ㎛. It is shown that tracking and erosion resistance depends on the median particle size of ATH. The tracking and erosion resistance decrease with increasing particle size.