Rheological properties of a high-density polyethylene (HDPE) subjected to thermal-mechanical degradation were compared with those of the HDPE degraded by the addition of small quantities of peroxides as free radical initiator. The high molecular weight fraction of HDPE changes quantitatively during processing, which has a remarkable effect on the theological properties of the material, when the MWD narrows and the HMW tail decreases. The molecular weight distribution, especially the highest molecular weight fraction, is concluded to be more important than the long chain for HDPE theology.