In this study, the influence of -nucleation agent (-NA) on the morphology and properties of multi-walled carbon nanotube (MWCNT) filled isotactic polypropylene (iPP) composites was explored in details. The results show that the incorporation of -NA has promoted the dispersion of MWCNT in the iPP matrix, which is profitable for improving the thermal stability and conductivity properties of MWCNT-iPP composites. Besides, the 0.05 wt % -NA nucleated samples exhibit higher impact toughness than that of un--NA-nucleated ones. Further SEM observations show that the morphology of MWCNT changes from large agglomerations to small clusters with doping of -NA. The main reason is that the incorporation of -NA (TMB-5) in MWCNT filled iPP matrix has led to the formation of a charge-transfer complex. Some of these clusters act as nucleation sites for inducing crystallization of spherulites, which have a compete growth with -NA induced crystals. Meanwhile, other clusters exist in the inter-lamella amorphous phase of crystals, some of them even combine two adjacent spherulites. Accordingly, a large conductive network comes into being. Based on the investigated results, a mechanism model is proposed. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013