Tensile behaviour and impact force-time curves of high density polyethylene/CaCO3 blends have been investigated. The results showed that there may exist a flexible interfacial area with lower stiffness than the matrix. The concept of yielding in the interfacial area was proposed to explain the fracture mechanism of HDPE/CaCO3 blends. Three types of fracture behaviour were assumed to be dependent on the surface-to-surface interparticle distance : (1) cavitation and crazing; (2) coexistence of cavitation and yielding in the interfacial area; (3) interfacial area yielding and matrix yielding. Based on the results, the interface conditions for brittle-ductile transition and core-shell morphology of CaCO3 particles were discussed.