The effects of the aspect ratio and the addition of multiwall carbon nanotubes (MWCNTs) on the morphological, electrical, mechanical and thermal properties of high-density polyethylene (HDPE)-based nanocomposites are investigated. The HDPE nanocomposites with various amounts up to 18.74 wt% of MWCNTs that have two different aspect ratios (AR) (200-400 (M58) and 500-3000 (M12)) were prepared by a melt mixing method, followed by compression molding. The results demonstrate that the addition of MWCNTs enhances the electrical, mechanical and thermal properties of the HDPE/MWCNTs nanocomposites. The HDPE nanocomposites with higher aspect ratio MWCNTs demonstrated a slightly better enhancement in the morphological and electrical properties than the other ones with lower aspect ratio MWCNTs. Additionally, Young's modulus and the thermal stability increased with increasing aspect ratio of MWCNTs.