The high-density polyethylene (HDPE) nanocomposites were prepared by using graphite nanosheets (GNS) and expanded graphite (EG), followed by foaming with subcritical CO2 used as an environmentally benign and nonflammable foaming agent. The partially exfoliated GNS and EG endow the prepared microcellular nanocomposite foams with high electrical conductivity, improved mechanical properties, as well as density reduction up to ca. 20 %. Interestingly, insulator-to-semiconductor transition of microcellular nanocomposite foams shifts to lower nanofiller content compared to that of bulk nanocomposites. Whether the nanofiller is GNS or EG, its incorporation leads to uniformly small cells, resulting in a remarkable enhancement in ductility without sacrificing toughness. It has demonstrated that foaming of HDPE nanocomposites with EG or GNS provides tough and lightweight microcellular foams, exhibiting the potential for use in conductive high-performance lightweight nanocomposite systems.