The addition of carbon nanotubes (CNTs) in polyacrylonitrile (PAN) precursor is an effective way to increase the electrical conductivity of derived carbon fibers. The electrical conductivity of 4.9 × 104 S/m for a PAN-based carbon fiber at room temperature increases to 9.4 × 104 S/m by adding 0.5 wt % CNTs. The measured conductivity for both PAN/CNT- and PAN-based carbon fibers monotonically increases as the temperature increases from 10 and 300 K. An attempt to explain the measured temperature dependences of electrical conductivities by various carrier transport models showed that a simple two-carrier model can give reasonable electron and hole mobility. A monopole antenna fabricated with PAN/CNT-based carbon fibers shows a gain of 2.3 dBi at 2.4 GHz, which is only 0.2 dB smaller than that of a reference (Cu-wire) monopole antenna. This result suggests the possibility of using PAN/CNT-based carbon fibers as antenna elements.