Polypropylene (PP) and nano-calcium carbonate (CaCO3) composites were prepared by melt mixing in a corotating twin-screw extruder. Transmission electron microscopy study and particle size analysis revealed the dispersion and the size distribution of CaCO3 in PP. With the increase of loading of filler, CaCO3 nanoparticles densely aggregated together and the dispersion of filler became bad. The fractal dimensions of the composites were determined using fractal concept. The fractal dimensions of D and D-k described the irregularities of the shape of an object and the distributions of particle populations, respectively. The D and Dk values were influenced by the content of filler, i.e., the D values increased, and the Dk values decreased with the increase of loading of filler. When the loading of filler was low, the values of D and Dk of PP composites differ slightly than the counterparts of PP/PP-g-MA (50 wt %) blend. For 20 wt %,, they were almost identical. This fact showed that the fractal dimension was correlated with the dispersion. (c) 2008 Wiley Periodicals, Inc.