The ability of stearate coated calcium carbonate nanoparticles to promote the nucleation of polypropylene (PP) was investigated systematically. The effects of surfactant coverage and CaCO3 particle concentration were explored using differential scanning calorimetry as well as optical and atomic microscopies. The results indicate that at the crystallization temperature of PP, a monolayer stearate coating remains as a rigid layer and provides a noticeable nucleating effect. Insufficient or excess coating diminishes the nucleating effect, the former because of the formation of agglomerates, and the latter by forming a soft layer at the PP/CaCO3 interface at high temperatures, leading to the weak nucleating ability. Monolayer-coated nanoparticles had the strongest nucleating effect. The crystallization temperature and crystallization rate increased with the concentration of the monolayer-coated nanoparticles up to 40 wt.%. (C) 2010 Elsevier Inc. All rights reserved.