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Partial graphitization of carbon nanofibers by high-temperature heat treatment can give improved composite properties. The intrinsic electrical conductivity of the bulk carbon nanofibers measured under compression is maximized by giving the fibers an initial heat treatment at 1500 °C. Similarly, for carbon nanofiber/polypropylene composites containing up to 12 vol% fiber, initial fiber heat treatments near 1500 °C give tensile modulus and strength superior even to composites made from fibers graphitized at 2900 °C. However, optimum composite conductivity is obtained with a somewhat lower heat-treatment temperature, near 1300 °C. Transmission electron microscopy (TEM) along with x-ray diffraction (XRD) explains these results, showing that heat treating the fibers alters the exterior planes from continuous, coaxial, and poorly crystallized to discontinuous nested conical crystallites inclined at about 25° to the fiber axis.