The effects of coupling agent, particle diameter (d), particle volume fraction (V-c), particle size distribution and average matrix ligament thickness ((T) over bar) on the impact strength of high density polyethylene (HDPE)/CaCO3 composites have been investigated. A coupling agent is required for dispersing the CaCO3 particles (d similar to 0.6-5.6 mum) without agglomeration in the HDPE matrix. The optimum weight ratio of coupling agent to CaCO3 particles for achieving the highest toughening efficiency is about 0.05, which is independent of d, V-c and size distribution of CaCO3 particles. HDPE/CaCO3 composites undergo a brittle-ductile transition in the range V-c = 0.1-0.3, and the maximum impact strength achieved depends on d and size distribution. Above the transition, the impact strength of the composite decreases with increasing V-c. In general, a HDPE/CaCO3 Composite with smaller d and broader size distribution has a higher toughness. A single brittle-ductile transition curve is not obtained when the impact strength is plotted against T. Shear yielding of the polymer matrix induced by debonding at the interface between CaCO3 and HDPE is shown to be the major toughening mechanism.