Three types of mineral fillers-talc, calcium carbonate (CaCO3), and kaolin (10-40 wt % filler loadings)-were compounded with polypropylene (PP) with a twin-screw extruder. The composites were injection-molded, and the effects of the filler loading on the mechanical, flow, and thermal properties for the three different types of filled composites were investigated. The aim was to compare their properties and to deduce prospective filler combinations that would yield hybrid PP composites in following studies. The results showed that in most cases, the strength and stiffness of the talc-filled PP composites was significantly higher than those of the CaCO3- and kaolinfilled PP composites. However, CaCO3, being a nonreactive filler, increased the toughness of PP. The kaolin-filled PP composites also showed some improvement in terms of strength and stiffness, although the increases in these properties were not as significant as those of the talc-filled PP composites. The effects of interfacial interactions between the fillers and PP on the mechanical properties were also evaluated with semiempirical equations. The nucleating ability of all three fillers was studied with differential scanning calorimetry, and the strongest nucleating agent of the three was talc, followed by CaCO3 and kaolin. (C) 2004 Wiley Periodicals, Inc.