We investigated changes in the hierarchical structures of polyethylene (PE) during tensile testing by means of time-resolved ultrasmall-angle scattering (USAXS), small-angle scattering, and wide-angle X-ray scattering. We discovered the enhancement of density fluctuation on the submicron scale by USAXS, which led to the generation of voids and necking. The spatial inhomogeneity of the stress fields associated with density or crystallinity fluctuation on the submicron scale induced the inhomogeneous flow during stretching. In other words, the change in the higher order structure than in the lamellar structure dominated the mechanical properties of PE. The enhancement of the fluctuation in linear low-density PE (LLDPE) was smaller than that in high-density PE (HDPE). Mechanical melting in the LLDPE suppressed the inhomogeneous flow and delayed the generation of voids and necking. As a result, the LLDPE exhibited two yield points on a S-S curve, while the HDPE exhibited one yield point.