Conductive nano-sized carbon black (CB) and carbon nanotube (CNT) are widely used in the polymer industry due to their excellent electrical and thermal conduction behavior and reinforcing ability. Herein, poly(dimethylsiloxane) (PDMS) nanocomposites filled with both conductive CB and CNT were fabricated using a planetary mixer and two-roll mill. Due to excellent thermal, electrical, and physical properties of CB and CNT, both the thermal and electrical properties of the composites and the dynamic mechanical properties were improved by the incorporation of CB and CNT into PDMS. The thermal conductivities of the composites linearly increased with filler concentration, and the electrical threshold was determined to be low. The rheological properties of the PDMS/CB and PDMS/CB/CNT nanocomposites were significantly influenced by filler incorporation.