Two kinds of carbonaceous particles, carbon black (CB) and carbon nanotubes (CNTs) were used as the filler to prepare biodegradable polylactide (PLA) composites for the study of synergistic effect of fillers on the electrical conductivity. The results reveal that the CB particles and CNTs are loosely entangled with each other in PLA matrix, instead of forming two independent phases. Therefore, there is no synergistic effect of these two fillers, and the ternary composites containing both the CB and CNTs present electrical conductivity ranged between those of the CB-based and CNT-based single-filler samples. However, after foaming with supercritical carbon dioxide, the ternary composite system reveals evident synergistic effect of fillers because the symbiotic structure between CB and CNTs favors the formation of the PLA cells with unbroken wall structure, which is propitious to the formation of the conductive filler networks with less defective structure in PLA bulk. As a result, the ternary composite foam shows better electrical conductivity than both the CB-based and CNT-based single-filler foams.