Abstract: Two commercial high-density polyethylene (PE-HD) materials specially used for geomembranes, TR131 and DQTG3912, were investigated comparatively in terms of their mechanical properties and chain structure. The HT 13C-NMR characterization result demonstrated that DQTG3912 had more short chains than TR131. The crystallinity of TR131 was analyzed by means of temperature rising elution fraction and successive self-nucleation and annealing, and the results indicated that TR131 had a lower degree of crystallinity. TR131 was found to have a higher relative content of low temperature fractions that DQTG3912, indicating a higher content of short-chain branching (SCB). DQTG3912 has less SCB at the low molecular weight parts but more SCB at the high molecular weight parts, which facilitated the formation of tie molecules and therefore improved its mechanical properties and resistance to environmental stress crack. The mechanical characterization results of DQTG3912 show good agreement with the analysis of its chain structure.