Nanostructured materials have attracted tremendous attention in past decades owning to their wide range of potential applications in many areas. In this study, novel conductive composite thermoplastic elastomers (CTPEs) were fabricated by using a copolymer-grafted multiwalled carbon nanotube (MWCNT) composite thermoplastic elastomer filled with varied amounts of unmodified MWCNTs as additional nanofillers. Rheological measurements and electrical conductivity tests were performed to investigate the viscoelasticity and electrical percolation behavior of these CTPEs, respectively. The incorporation of unmodified MWCNTs can significantly increase the electrical conductivity of these CTPEs, and the electrical conductivity percolation threshold was determined to be 0.34 wt %. The macroscopic mechanical properties of these CTPEs can be conveniently adjusted by the content of unmodified MWCNTs; for example, the strain-hardening behavior can be significantly enhanced with the incorporation of unmodified MWCNTs. This design concept can be generalized to other conductive composite elastomeric systems.