Calcium carbonate-filled syndiotactic poly-(propylene) (CaCO3-filled s-PP) was prepared in a self-wiping, co-rotating twin-screw extruder. The effects of CaCO3 of varying particle size (1.9, 2.8 and 10.5 mum), content (0-40 wt %), and type of surface modification (uncoated, stearic acid-coated, and paraffin-coated) on the crystallization and melting behavior, mechanical properties, and processability of CaCO3-filled s-PP were investigated. Non-isothermal crystallization studies indicate that CaCO3 acts as a good nucleating agent for s-PP. The nucleating efficiency of CaCO3 for s-PP was found to depend strongly on its purity, type of surface treatment, and average particle size. Tensile strength was found to decrease, while Young's modulus increased, with increasing CaCO3 content. Both types of surface treatment on CaCO3 particles reduced tensile strength and Young's modulus, but improved impact resistance. Scanning electron microscopy (SEM) observations of the fracture surfaces for selected CaCO3 filled s-PP samples revealed an improvement in CaCO3 dispersion as a result of surface treatment. Finally, steady-state shear viscosity of CaCO3 filled s-PP was found to increase with increasing CaCO3 content and decreasing particle size. (C) 2004 Wiley Periodicals, Inc.