The effects of the crystallinity and a radiation crosslinking on the high-density polyethylene (HDPE) with a nucleating agent were investigated. We found the optimum conditions for the stretching process according to the addition of various quantities of a nucleating agent (Millad3988). The pores of a HDPE membrane were affected by the crystallinity of the polymer, and the crystallinity of the polymer was changed with an increase thermal ageing temperature. Thermal ageing treatment of the HDPE film was conducted in an oven at 110–135°C for 5–60min. When the conditions for the annealing were fixed at 125°C and 40min, we obtained the highest crystallinity. Also, the resulting mechanical properties of the irradiated HDPE separators were analyzed.