The main aim of this work was to study and compare the mechanical and thermal properties of hybrid polypropylene (PP) composites and single-filler PP composites. With two main types of mineral fillers-calcium carbonate (CaCO3) and talc-PP composites of different filler weight ratios (talc/CaCO3) were compounded with a twin-screw extruder and then injection-molded into dumbbell specimens with an injection-molding machine. Tensile, flexural, and impact tests were performed to determine and compare the mechanical properties of the hybrid and single-filler PP composites. A synergistic hybridization effect was successfully achieved; the flexural strength and impact strength were highest among the hybrids when the PP/talc/ CaCO3 weight ratio was 70:15:15. The nucleating ability of the fillers and its effects on the mechanical properties were also studied with differential scanning calorimetry. Because of the influence of talc as the main nucleating agent, the hybrid fillers showed significant improvements in terms of the nucleating ability, and this contributed to the increase in or retention of the mechanical properties of the hybrid composites. (C) 2004 Wiley Periodicals, Inc.