Nano-CaCO3/polypropylene (PP) composites modified with polypropylene grafted with acrylic acid (PP-g-AA) or acrylic acid with and without dicumyl peroxide (DCP) were prepared by a twin-screw extruder. The crystallization and melting behavior of PP in the composites were investigated by DSC. The experimental results showed that the crystallization temperature of PP in the composites increased with increasing nano-CaCO3 content. Addition of PP-g-AA further increased the crystallization emperatures of PP in the composites. It is suggested that PP-g-AA could improve the nucleation effect of nano-CaCO3. However, the improvement in the nucleation effect of nano-CaCO3 would be saturated when the PP-g-AA content of 5 phf (parts per hundred based on weight of filler) was used. The increase in the crystallization temperature of PP was observed by adding AA into the composites and the rystallization temperature of the composites increased with increasing AA content. It is suggested that the AA reacted with nano-CaCO3 and the formation of Ca(AA)2 promoted the nucleation of PP. In the presence of DCP, the increment of the AA content had no significant influence on the crystallization temperature of PP in the composites.