Different blends of high-density polyethylene (HDPE) with calcium carbonate (CaCO3) were mechanically tested under uniaxial tension with or without poly(ethylene-co-octene) elastomer grafted with maleic anhydride (POEg), as an impact modifier. In some materials, the surface of the CaCO3 was treated with an amino acid and in others the mineral particles were left untreated. The stress-strain behavior were determined at constant true strain rate by using the VideoTraction(R) system. Also, the volume changes upon stretching was assessed by means of the video extensometer and correlated with X-ray densitometry measurements. The dependence of modulus, yield stress, and cavitation is shown to depend on the relative percentage of the three constituents. In particular, the cavitation rate increases markedly with the CaCO3 content and decreases with the POEg content. By contrast, the surface pretreatment of the CaCO3 particles appear to be of much lesser importance.