Abstract: Quaternary composites consisting of two different types of filler (carbon black and carbon fiber) and binary polymer (polyethylene and polypropylene) were prepared by melt blending. The microstructure of the conductive networks and conductivity properties of the composite were investigated. The results show that the double---percolated networks, as well as the mutual bridging between carbon fiber and conductive high density polyethylene (HDPE) phase region reduce the percolation threshold and improve the conductivity. The volume resistivity of quaternary composites decreases by 1～5 orders of magnitude at the same filler content compared to CB/HDPEPP and CBCF/HDPE ternary system. Meanwhile, the negative temperature coefficient effect is inhibited and the stability of the electrical properties is raised, due to the existence of the double---percolated networks. Compared with CBCF/HDPE ternary system, the NTC effect of CBCF/HDPEPP system decreases from 2 orders of magnitude to 0.6 orders of magnitude, while the characteristic relaxation time increases to 2370s from 951s.