Abstract: The study of the resistivity temperature dependence of polyethylene-carbon-black composites mixed with different concentrations of carbon fiber is reported. Carbon fibers provide charge transport over large distances, whereas carbon blacks not only provide charge transport over relatively small distances but also improve the interfiber contacts through carbon black particle bridges. Incorporation of carbon fibers into polyethylene-carbon black composites can substantially improve the conductivity level of these systems. Only charge transport over relatively small distances is affected by the increasing temperature before PTC transition. Through black particles bridging neighbouring carbon fibers, carbon fiber conductive network can compensate for somewhat conductivity losses resulting from the increasing interparticle distances of carbon black. Resistivity of composites before PTC transition is thus more and more insensitive to temperature change due to the increase of carbon fiber loading. However, in the temperature range of polyethylene melting point, charge transports over large and small distances are both destroyed in the composites. In this case resistivity increases drastically, giving rise to an extremely large PTC anomaly effect. So the positive temperature coefficient of resistance (PTC) effects become stronger, and the PTC transition temperature is increased as concentration of carbon fiber in polyethylene-carbon-black composite.