An internal mixer was used to prepare polycarbonate (PC)-based nanocomposites containing carbon fibers, carbon nanofibers (CNF), and mixtures of the two fillers. The influence of the filler volume fraction, the relative amounts of the two fillers, and the filler orientation relative to the direction of heat flow on the thermal conductivity was examined. Filler orientation was obtained by the extrusion of strands of the nanocomposite. The thermal conductivity was measured using a steady-state heat conduction technique. The CNF were fragile, and their aspect ratio could be decreased during processing. In general, the composite thermal conductivity increased with increasing filler content. Fiber alignment in the heat flux direction resulted in a significant increase in thermal conductivity. Mixing of nanofibers with microfibers resulted contacts between the microfibers. This, together with fiber alignment provided large increases in the thermal conductivity. POLYM. ENG. SCI., 48:2474-2481, 2008. (C) 2008 Society of Plastics Engineers