In this work, mechanical and electrical properties of graphenes (GP)/carbon nanotubes (CNTs) co-reinforced high density polyethylene (HDPE) matrix composites were studied. The microstructure, morphologies, and electric properties of the composites were evaluated by XRD, TEM, and 4-probe methods, respectively. It was found that the electric resistivity of 0.5 wt %-GP/HDPE was immeasurable, and 2.0 wt %-CNTs/HDPE showed high resistivity ($6.02{ imes}10^4{Omega}{cdot}cm$). Meanwhile, GP (0.5 wt %)/CNTs (2.0 wt %)/HDPE showed excellent low resistivity ($3.1{ imes}10^2{Omega}{cdot}cm$). This result indicates that the co-reinforcement systems can dramatically decrease electric resistivity of the carbon/polymer nanocomposites.