PVC filled with carbon black (CB) composition were prepared by a conventional melt-mixing method. The effects of CB on the negative-temperature-coefficient (NTC) effect and the resistivity-temperature characteristic of PVC/CB composites were studied. Our present research suggests that percolation threshold can be obtained for the content of CB 10%. The reasons of NTC phenomenon were also investigated from point of view of polymer structure. We also discussed relationship between the current and voltage, that is, the I-U characteristic of the composites can be divided into two regions, ohmic and nonohmic according to the different content of CB and voltage.