Calcium carbonate (CaCO3)/high density polyethylene (HDPE) composites were prepared in a HAAKE twin screw extruder, using the experimental conditions defined by the factorial experimental design presented in a prior study. In this study, the effect of different grades (Ca-1 and Ca-2) and CaCO3 content (varying from 0 to 15 wt %) on the mechanical, rheological, thermal, and morphological properties was evaluated. The results showed that the addition of the filler provoked a decrease on the impact strength, stress at break, and yield stress properties in relation to the pure HDPE. A consequent increase on the modulus of elasticity, indicating an increase on the rigidity of the composite, was observed. It was also verified a tendency to increase the toughness and the viscosity of the composites as CaCO3 was added. Scanning electron micrographs showed that as the filler was incorporated to HDPE matrix, CaCO3 particles tended to agglomerate, especially that grade constituted of particles of smaller sized The thermal analysis showed that the addition of mineral filler caused a decrease on the crystallinity degree. (c) 2006 Wiley Periodicals, Inc.