Using cylindrical specimens with short gauge length, bulk cavitation is successfully generated in high-density polyethylene copolymers (HDPE) under tensile loading. The bulk cavitation is generated without the presence of sharp notch, and therefore, crack growth is not involved in the rupture process. The bulk cavitation is investigated in two types of HDPE that have different molecular characteristics. Test results indicate that bulk cavitation can be introduced in both the types of HDPE, for which the loading level is most easily identified using plots of true axial stress versus stroke. The main difference in the rupture process occurs during the postcavitation stretch. Using specimens of sufficiently long gauge length, constitutive equations for the HDPEs are determined and applied to finite element modeling of specimens with short gauge length. The results show that the hydrostatic stress level in specimens of short gauge length is about twice as high as that in specimens of long gauge length, supporting the conclusion on bulk cavitation from rubber testing. For HDPE, this study shows that the rise of hydrostatic stress level leads to the suppression of necking, to allow the bulk cavitation to dominate the deformation process. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers