Electrically conductive polypropylene (PP)/multi-walled carbon nanotube (MWCNT) composites containing electrically inert particulate filler calcium carbonate (CaCO3) have been prepared in a rotational rheometer. A significant reduction in electrical resistivity was found with the addition of CaCO3. The concept of effective concentration of MWCNTs is proposed to quantitatively evaluate the effect of CaCO3. master curve was achieved by plotting electrical conductivity (or resistivity) data of various composite systems, with or without CaCO3, against their effective volume fraction of MWCNT, validating the concept of effective concentration. Similar results were obtained from investigations on MWCNT composite systems of different inert fillers, including talc and wollastonite, and host polymers. such as polyoxymethylene and polyamide, demonstrating the generality of the present observation. (C) 2008 Elsevier Ltd. All rights reserved.