Nano-/microcomposites based on polypropylene/montmorillonite/calcium carbonate were prepared by melt mixing. Their structures and properties were characterized by small-angle X-ray diffraction, thermal analysis, and rheological measurements. The intercalation degree was found to be dependent on the compatibilizer content and the processing temperature. The addition of the organoclay slightly increased the melt crystallization temperature of polypropylene, acting as nucleating agents, and improved the degree of crystallinity. The rheological tests showed that nanocomposites increased the complex viscosity when compared with the microcomposites with the same filler content and exhibited a pronounced shear-thinning behavior in the low frequency range. A Carreau-Yasuda model was used to model the rheological behavior of these materials. The nano-/microcomposites showed a significant improvement (about 50%) of the Young's modulus, when compared with microcomposites with the same filler content due to the intercalation or exfoliation of the organoclay anti the enhanced degree of crystallinity. Moreover, some formulations showed an enhancement of elongation at break and ultimate strength. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3360-3367, 2009