The primary aim of this paper is to provide an insight on the effect of the distribution of calcium carbonate nanoparticles (nano-CaCO3) on the isothermal crystallization kinetics in Polypropylene (PP)/ethylene-propylene-diene terpolymer (EPDM)/nano-CaCO3 composites prepared by different compounding procedures. PP/EPDM/CaCO3 composites were prepared by two compounding procedures (direct compounding: mixed all three solid materials together; multistep compounding: the melted EPDM/CaCO3 master batch in a single screw extruder blended with melted PP in twin-screw extruder via injecting into the twin-screw extruder from a lateral port at the melting section of the twin-screw extruder). Morphological observation showed that abundant CaCO3 particles concentrated around EPDM dispersed phase in the multistep compounding composite, essentially different from the respectively dispersed morphology of CaCO3 particles and EPDM domains in the matrix for the direct compounding composite. Moreover, better dispersion of CaCO3 particles in the multistep compounding composite was observed comparing to the direct compounding composite. Futhermore, a pronounced improvement of the crystallization half time (t1/2), rate of crystallization (G) was achieved in the multistep compounding composite, which may originate from the better dispersion of CaCO3 particles providing larger nucleation density and the collaborative nucleation of EPDM and CaCO3 during the isothermal crystallization.