The effect of particle size distribution on the brittle ductile transition of high density polyethylene (HDPE)/CaCO3 composites is studied. Fu and coworkers reported that the brittle ductile transition curves of HDPE/CaCO3 composites obtained by plotting impact strength against matrix ligament thickness do not give a master curve. In this work, the splitting of brittle ductile transition master curve of the composites as matrix ligament thickness is smaller than its critical value at brittle ductile transition is demonstrated to be attributable to the effect of CaCO3 particle size distribution on matrix ligament thickness. The critical matrix ligament thickness criterion proposed by Wu is thus valid for the brittle ductile transition of HDPE/CaCO3 composites when the effect of particle size distribution on matrix ligament thickness is considered. The relations of critical particle size and of critical particle volume fraction at brittle ductile transition to particle size distribution are analyzed, respectively. A narrow particle size distribution is favourable to the enhancement of the toughness of polymer composites.