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The impact properties of glass-fibre/impact-modifier/polypropylene (GF/IM/PP) hybrid composites were characterized using a number of impact test methods. For the IM/PP blends, the impact fracture toughness can be measured using linear elastic fracture mechanics (LEFM) approach. For the GF/IM/PP hybrids, due to their non-compliance with LEFM, the essential work of fracture approach was employed. The impact properties of the IM/PP blends increased with IM concentration, while that of the GF/IM/PP hybrids did not change very much with IM content. It was concluded that cavitation of the PP matrix around the IM particles was the major toughening mechanism in the IM/PP blends. However, in the GF/IM/PP hybrids, the toughening effect due to cavitation was suppressed due to the introduction of short glass fibres (≈15 vol%). It is believed that the local stress in the matrix was relieved by fibre/matrix debonding of the relatively weak fibre/matrix interface. Thus, the presence of the IM particles was rendered insignificant in the GF/IM/PP hybrids.