The separator made from the blends of high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE) was prepared by wet processing to use as Li-ion secondary battery. We investigated effects of the blending of the polymers and the film extension on the mechanical properties of the separator. The mechanical strength of separator increased with increasing molecular weights and contents of UHMWPE, for instance about $1000 kg/ extrm{cm}^2$ with the five times extended film of 6 wt% UHMWPE. The pores of the separator were very uniform with the size of 0.1~$0.12mu extrm{m}$. The shut-down characteristic quickly increased at around $130^{circ}C$ and the fusion temperature was $160^{circ}C$, so it could be applied to the lithium ion secondary battery.