A novel simultaneous, in situ milling and coating method carried out in a fluid energy mill (FEM) is applied for the first time to prepare nanoparticle-coated CaCO3 (CC) additives for polymer composite materials. Simply milled (without coating) CC particles and as-received CC particles were used as references for comparison. The effects of the grinding pressure and the fraction of the nanoparticles on the size and flowability of CC particles were studied. The composites made with polypropylene (PP) and this specially prepared CC have higher elongation at break, elastic modulus, and impact strength, compared with the PP filled with uncoated CC. The thermal and thermo-oxidative stabilities of PP are improved as well by introducing the milled and nanoparticle-coated CC. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers