This paper is focused on the quantitative characterization of cavitation developing in unfilled and nano-CaCO3 filled high density polyethylene (HDPE) subjected to tensile test. In particular, a methodology to reveal the voids, take into account their localization, and evaluate their characteristics (density and shape), is presented. For this, post-mortem investigations coupling a cryosectioning method and a pressure-controlled scanning electron microscope (PC-SEM) were conducted. In the two materials, whatever the active cavitation mechanism (crazing in neat HDPE or debonding in CaCO3/HDPE system), it was shown that the cavity density and the cavity height/width ratio (shape factor) increase with the deformation state. These two parameters are also influenced by the macroscopic stress triaxiality generated by the necking process. Indeed, when the intensity of the triaxiality is high, the cavity density and the cavity shape factor significantly increase from the skin to the core of the specimen. POLYM. ENG. SCI., 50:278-289, 2010. (C) 2009 Society of Plastics Engineers