Although the presence of carbon nanotubes (CNTs) makes polypropylene (PP) electrically conductive, the resulting PP/CNT binary nanocomposites become brittle limiting their practical applications. To toughen PP/CNT nanocomposites, calcium carbonate (CaCO3) inorganic nanoparticles are melt-compounded with PP and CNTs components to fabricate electrically conductive and tough PP/CNT/CaCO3 ternary nanocomposites. The PP/CNT nanocomposites have a relatively large percolation threshold of 6.2 wt %, which reduces to 5.6 wt % by the addition of 30 wt % of pristine CaCO3, and further to 3.6 wt % in the presence of 30 wt % of modified CaCO3. Simultaneously, the electrically conductive PP/CNT nanocomposites are efficiently toughened by the CaCO3 nanoparticles, and the notched impact strength increases from 16.0 to 33.1 KJ/m(2) by compounding 30 wt % of modified CaCO3 with PP/9 wt % CNT components. The dual roles of CaCO3 in volume-exclusion and toughening are well demonstrated.