Abstract: Conductive properties of polymer composites filled with two kinds of carbon materials (carbon nanotubes and carbon black or graphite) are introduced and the conductive mechanism is expounded. The calculating model of electrical percolation threshold based on linear blending rule and excluded volume theory are separately given and the difference between calculating values and experimental data is discussed. Using the excluded volume theory, a calculating model of percolation threshold of polymer filled with two kinds of carbon materials is derived and compared with that given in a reference. The newly-derived model indicates that the percolation threshold of polymer filled with two kinds of fillers is a weighted average value of those filled with one kind of filler and the arrangement pattern of carbon nanotubes and carbon black particles in the mixed-fillers filled polymer is different from that of carbon nanotubes or carbon black particles filled alone and this difference in arrangement causes the difference between calculating values and experimental data. A train of thought, “equivalent substitution of length of removed carbon nanotubes by carbon black particles”, is proposed, from which a new calculating model of percolation threshold of polymer filled with two kinds of carbon material is derived. The calculating values are consistent with experimental data.