In this study, we performed the crystallization of carbon nanotube (CNT)/isotactic polypropylene (iPP) and graphene nanosheet (GNS)/iPP composites with very high nanofiller loadings; these are frequently used in polymer composites for electromagnetic interference shielding and thermal conductivity. Rheology testing indicated that both the high-loading CNTs and GNSs formed dense networks in the iPP matrix, and transmission electron microscopy showed that their connection types were completely different: the CNTs contacted one another in a dot-to-dot manner, whereas the GNSs linked reciprocally in a plane-to-plane manner. The carbon nanofiller networks brought about two opposite effects on iPP crystallization: a nucleation effect and a confinement effect. The CNT network showed a stronger nucleation effect; however, the CNT network also revealed a more powerful confinement effect because the CNT network was denser than the GNS network. With increasing content of the carbon nanofillers, the crystallization rates of both the CNT and GNS composites first increased, then decreased, and showed a very high saturation concentration at 50 wt %; this resulted from the competitive relationship between the nucleation effect and confinement effect. The crystallization was facilitated when the carbon nanofiller concentration was below saturation, where the nucleation effect invariably played a dominant role. Although the crystallization was depressed when the carbon nanofiller concentration was above saturation, the nucleation effect was subdued, and the confinement effect was extensive. Compared to the GNS/iPP composites, the CNT/iPP composites showed a more depressed crystallization. The suppression mechanism is discussed with consideration of the local topological structure constructed by those two carbon nanofillers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39505.