In this work, the properties of high density polyethylene (HDPE)/graphite nanosheets (GN) and HDPE/carbon black (CB) nanocomposites prepared by melt mixing were studied. GN-filled HDPE nanocomposites exhibited very low percolation threshold (ca. 6 wt%) as compared with that of HDPE/CB (ca. 22 wt%). Moreover, the impact strength and processing properties of HDPE/GN nanocomposites were superior to those of HDPE/CB within a certain content of fillers. The large aspect ratio of GN plays an important role in reducing the percolation threshold of HDPE/GN nanocomposites. Furthermore, the tensile fracture morphology revealed the formation of continuous conducting networks after the content of GN reached the critical percolation threshold. Differential scanning calorimetry (DSC) analysis illustrated a decreasing degree of crystallinity of HDPE/GN nanocomposites, while the crystalline temperature varied slightly. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers