The toughness and the rheology of polypropylene (PP)-calcium carbonate (CaCO3) nanocompounds using stearic acid as an interface modifier were studied in this work. Compounding of the nanocomposites was carried out with a twin-screw extruder. The benchmark samples (untreated) and stearic acid-treated CaCO3 nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy. The nanocompounds were characterized by impact test, tensile test, scanning electron microscopy (SEM), rheological analysis, and differential scanning calorimetry (DSC). The elongation-at-break and impact resistance were increased in nanocompounds with interface modifier (stearic acid in hopper of the extruder). Nanocompounds with stearic acid showed the best dispersion state. Stearic acid helps to reduce complex viscosity acting as a lubricant, reducing frictional forces between nanoparticles of calcium carbonate (NCC) and PP chains. Nanocompounds with better dispersion state had crystallization temperatures very similar to the PP homopolymer. POLYM. ENG. SCI., 59:E279-E285, 2019. (c) 2019 Society of Plastics Engineers