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（tm） 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.
