Three types of polypropylene (PP) with different intrinsic toughness were used to study the, mechanical properties and morphologies of the PP composites filled with single-filler and hybrid-filler of calcium carbonate particles. The calcium carbonate particles used were with average particle sizes of 25 mu m (CC25), and 0.07 mu m (CC0.07), respectively. A hybrid-filler CaCO3 named CC25/CC0.07 was used as a mixture of CC25 and CC0.07 (CC25/CC0.07 weight ratio = 1:1). It was found that the type of PP and the particle size of inorganic filler were the two important factors for the determination of mechanical properties of the composites. And the general mechanical properties of the composites filled with hybrid-filler CaCO3 were better than those of the composites filled with single-filler CaCO3, but the synergistic hybridization effect of the hybrid-filler CaCO3 did not exist. The major toughening mechanism of the PP/CC25 composites was the cavitation of the matrix caused by CC25, and the major toughening mechanism of the PP/CC0.07 composites was the pinning effect introduced by CC0.07. For the PP/CC25/CC0.07 composites, the cavitation of the matrix caused by CC25 and the pinning effect introduced by CC0.07 existed simultaneously. And when the intrinsic toughness of the matrix was large enough, the major factor to toughen PP was the pinning effect introduced by CC0.07, otherwise the major factor to toughen PP was the cavitation of the matrix caused by CC25.