Polypropylene (PP)/multi-walled carbon nanotube (MWCNT)/calcium carbonate (CaCO3) composites are prepared by melt mixing using two types of CaCO3 of different sizes. The electrical resistivities of the composites with the two types of CaCO3 are all lower than those of the corresponding PP/MWCNT composites at various MWCNT loadings (1 wt%-5 wt%). The morphology of the composites is investigated by field emission scanning electron microscopy (FESEM). The crystallization behavior of PP in the composites is characterized by differential scanning calorimetry (DSC). The storage modulus, as measured by dynamic mechanical analysis (DMA), increases significantly by the presence of CaCO3.