Precipitated CaCO3 (PCC)/High Density Polyethylene (HDPE) composites were prepared on a twin screw mixer-single screw extruder with a particle content of 10 vol%. The average particle size was 70 nm. The influence of surface treatment of the particles, with and without stearic acid (SA), on the physico-mechanical and theological properties was studied. The experiments included tensile tests, impact tests, differential scanning calorimetry (DSC), microscopy and theology experiments. The addition of 10 vol% calcium carbonate to HDPE causes a rise in Young's modulus and yield stress of its composites and is accompanied by a sharp drop in impact strength. The addition of SA has the effect of slightly decreasing both Young's modulus and yield stress of the composites compared to the uncoated PCC composites, while the impact strength progressively increases. During the tensile test filled HDPE composites showed stress whitening zones appear and develop along the gauge length. Volume measurements during tensile tests showed an increase in volume strain with deformation, due to the matrix-particle debonding phenomenon, while pure HDPE showed actually a decrease in volume with elongation. At constant deformation, for the composites with coated PCC, it can be observed that an increase in the SA content leads to a slight decrease in volume change. The microscopical evaluation showed cavities and voids due to debonding and deformation bands in the stress whitened areas. DSC experiments have shown that uncoated PCC particles have a very small nucleating effect on HDPE. (C) 2004 Published by Elsevier Ltd.