Propylene-ethylene random copolymer (PEC) elastomer was used to toughen high-density polyethylene (HDPE). The morphology, thermal behavior, rheological, and mechanical properties of HDPE/PEC blends were investigated. Scanning electron microscopy (SEM) results revealed that PEC-rich domains dispersed evenly in the HDPE matrix, and HDPE/PEC blends were immiscible. Rheological measurements indicated that the HDPE/PEC blends exhibited higher melt strength and viscosity than neat HDPE, which was beneficial to foaming and blow molding of HDPE. From the results of DSC, PEC elastomer inhibited the crystallization and reduced the degree of crystallinity of HDPE. It was noteworthy that the significant enhancement of the toughness of HDPE was obtained by adding PEC elastomer. The maximum elongation at break (684.6%) and impact strength (35.3 kJ m−2) were achieved for the HDPE/PEC blends, which increased by 110% and 240%, respectively, compared with neat HDPE. Therefore, PEC elastomer could be used as an effective impact modifier for HDPE. [ABSTRACT FROM AUTHOR]