Differential scanning calorimetry, wide-angle X-ray diffraction, small-angle X-ray scattering, and transmission electron microscope are employed to study the microstructure of biaxially self-reinforced high-density polyethylene (HDPE) prepared in uniaxial oscillating stress field by dynamic packing injection molding. The results indicate that the biaxial self-reinforcement of HDPE is mainly due to the existence of interlocking shish-kebab morphology (i.e., zip fastener structure), along with the orientation of lamellae and molecular chains and the enhanced crystallinity. (C) 2004 Wiley Periodicals, Inc.