As a kind of industry dust and one source of PM2.5, carbon ash, mainly composed of amorphous carbon and ZnO, is urgently to be utilized. In this work, carbon ash and acid treated carbon ash were used as novel conductive filler to prepare high‐k composites with high density polyethylene (HDPE). When the content of carbon ash was up to 20 wt %, carbon ash/HDPE composite possessed of a high dielectric constant of 83 at l000 Hz, which was about 36 times of pure HDPE. Using acid treated carbon ash as filler, the composite would have an enhanced dielectric constant of 458, when the filler content was 20 wt %. This was caused by the different conductivity of fillers before and after treated by acid. Besides, 3‐aminopropyltriethoxysilane was employed to decrease the dielectric loss of the composites through modifying those fillers. The results showed that carbon ash was good candidate to prepare high‐k composites. © 2017 American Institute of Chemical Engineers Environ Prog, 36: 1719–1724, 2017