A separator made of the polymer blends of high density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) is prepared by a wet process for the Li-ion secondary battery. An investigation is made of the effect on the mechanical properties of the separator by the blending of the polymers and the drawing conditions. The mechanical strength is increased with increasing molecular weight and content of UHMWPE polymer. A film containing 6wt.% UHMWPE has a tensile strength of about 1000kgcm−2 at a draw ratio of 5. The pores of the separator are very uniform with a size of 0.1–0.12μm. The shut-down (SD) characteristics increase rapidly in the vicinity of 130°C and the fusion temperature is 160°C. These features suggest that the separator is applicable to the Li-ion secondary battery.