Polymeric insulators role in power systems and insulation is one of the most important subjects in high voltage applications. This study investigates the effect of alumina trihydrate (ATH) loading on electrical and flammability properties of polypropylene (PP)/ethylene propylene diene monomer (EPDM) blend. ATH fillers were incorporated into the matrix to improve the tracking and flammability resistance of the composites. PP/EPDM (60:40) was used as matrix and compounding of ATH filled PP/EPDM composites was done using a twin-screw extruder. Electrical and flammability tests of composites were performed. The results show that flammability decreased with increasing ATH loading in PP/EPDM composites. The effects of electrical stresses are investigated by measuring the carbon tracking (CT) and leakage current (LC) on the surface of composites. The CT and LC of PP/EPDM/ATH composites were found to decrease with increasing ATH content; however calculating the normalized degradation index (NDI) illustrated that all the sample were in accepted range. Field emission scanning electron microscope was used to relate the electrical properties with the morphological analysis, which showed that addition of ATH prevents the degradation of the matrix. POLYM. ENG. SCI., 54:493–498, 2014. © 2013 Society of Plastics Engineers