The electrical conductivity of polymers can be increased by the addition of conductive fillers, including forms of carbon fibers and carbon black. The resulting composites can be used in applications where metals have typically been the materials of choice. The advantages of using these materials include lighter weight, resistance to corrosion, and the ability to be readily adapted to the needs of a specific application. One of the most significant applications for conductive polymer composites are conductive carbon-polymer composite electrodes. As many properties such as conductivity, mechanical integrity, low permeability, electrochemical activity, and stability in the electrolytes are required of materials to be used as electrodes, so "material selection" plays a crucial role in fabricating these materials. In this work it was found that high density polyethylene (HDPE)/ethylene-propylene-diene monomer(EPDM) blend ratio (70/30) has lower percolation threshold and volume resistivity than individually carbon black filled HDPE and EPDM due to "double percolation" effect. Carbon fibers were also added to the polymer-carbon black mixtures to enhance the conductivity and mechanical properties. The electrical conductivity of composites with different ratios of carbon black (CB) content to carbon fiber (CF) content was studied. The CB content is the main factor to determine the resistivity of the composites filled with CB and CF. Mechanical properties, including tensile strength, elongation at break, and impact strength of the conductive composites were evaluated. The results showed that incorporation of CB and CF in the composites will enhance tensile strength, but decrease elongation at break and impact strength of the composites. In mechanical properties, CF content has a greater effect than CB content. From the comparison of the resistivity and mechanical properties of the composites filled with CB and CF with that of the composites filled with CB only, it is concluded that using CF as a substitute for part of the CB in CB-filled composites can enhance electrical and mechanical properties.