Conductive properties of multiwalled carbon nanotubes(MWNTs) filled high-density polyethylene(HDPE) composites prepared by melt blending were studied. The results showed that as the MWNTs concents reach to 3%(mass fraction), the electrical conductivity of the composites changes from insulator to semiconductor, possessing resistivtiy of 10 10 Ω·cm. While as the MWNTs concents reach to 5%, the composites become a good conductor. The resistivity-temperature characteristic of the composites exhibits that with the increasing of the temperature, the resistivity of the composites first decreases, while increases abruptly when the temperature approaches melting point(t_m) of the matrix, indicating an existence of the negative temperature coefficient(NTC)-positive temperature coefficient(PTC) effect. It should be noted that this phenomenon can not be found in polymer-based composites filled with other conductive particles, such as carbon black, graphite, carbon fiber.