Highly oriented samples from ultra-high molecular weight and normal high-density polyethylene (UHMWPE, HDPE) were prepared under the same experimental conditions. The morphology of the UHMWPE, investigated by transmission electron microscopy, was found to be oriented micellar, while the HDPE samples had a shishkebab morphology. Tensile test experiments under cyclic loading conditions exhibit a strain-hardening effect (increase in stress for the onset of plastic deformation) for HDPE, while the UHMWPE shows a decrease in Young’s modulus, which is attributed to similar molecular mechanisms as for the Mullins effect in particle-filled elastomers.