Abstract: As a kind of fibrillar conductive filler，multi-wall carbon nanotubes (CNTs) are int roduced into carbon black (CB)/polypropylene (PP) composites，so as to combine their different conductive mechanisms. The experimental results reveal that CNTs can form bridges between CB particles and result in a remarkable decrease of volume resistivity of CB/PP composites. CB∶CNTs=19∶1 is the optimum proportion for CB-CNTs/PP composites.The conductive percolation threshold of CB-CNTs (19∶1)/PP composite is much lower than that of CB/PP composite. When the total volume fraction of the conductive filler is 4.76%，due to the addition of CNTs，the volume resistivity of the composites can decrease from 10⁹Ω·cm to 10⁵Ω·cm，the positive temperature coefficient(PTC) intensity of CB/PP composites also decreases from 6.10 to 1.48，and the peak temperature of PTC transition increases from 166℃ to 174℃. The crystallizing point of PP rises by 12℃ even if at a very low CNTs content due to CNTs‘ higher nucleation than CB. The mechanical properties of the composites remarkably decrease with an increase of CB or CB-CNTs filler volume fraction. To reach the same volume resistivity，the conductive filler volume fraction of CB-CNTs/PP composite is much lower than that of CB/PP composite，so the higher mechanical properties of CB-CNTs/PP composite can be maintained by the addition of CNTs.