Carbon fiber (CF) filled low-density polyethylene (LDPE) composites were prepared by the conventional melt-mixing method. The distribution of CF in the composite was studied by wide-angle X-ray diffraction (WAXD) and scanning electron microscope (SEM) observations. A phenomenological model was proposed to illustrate the resistivity-temperature behavior of CF-filled semicrystalline composites. The effects of the content and aspect ratio of CF on the positive temperature coefficient (PTC) and the room temperature resistivity were elucidated. A balance between the PTC intensity and the room-temperature resistivity can be achieved by using a mixture of CFs with low and high aspect ratios. The negative temperature coefficient (NTC) phenomenon can be effectively eliminated by crosslinking under gamma-ray radiation, and the crosslinked composite exhibits a higher PTC intensity and PTC transition temperature than the noncrosslinked counterpart. (C) 2003 Wiley Periodicals, Inc.