High Density Polyethylene (HDPE) was melt extruded with different amounts of expanded graphite (EG) based masterbatches. Conductive composites were obtained by diluting PE and PS masterbatches with 60 wt% content of expanded graphite. These masterbatches were readily dispersed into the molten HDPE matrix yielding well-dispersed HDPE/EG nanocomposites which couldn't be done by direct melt extrusion process under the same conditions. Electrical conductivity measurements showed a reduced percolation threshold by this masterbatch filling technique while the resulting composites were 2-3 orders of magnitude lower than that of direct melt extrusion because EG sheets were effectively encapsulated by PE or PS carriers in these masterbatches which leads to a better EG dispersion in composites. Both scanning electron microscopy (SEM) and X-Ray diffraction (XRD) proved an excellent dispersion of EG in polymer matrix with the worm-like structure tended to break into pieces under intensive rolling. The improvements in mechanical and thermal properties have been studied for the nanocomposites as prepared by masterbatch process. The results depended greatly on the dispersion of EG and the compatibility between masterbatch and HDPE matrix.