Polyethylene (PE) membranes having various porosities are used as microfilters and separators in lithium ion batteries. Membranes having a high porosity are required for use as separators in a large scale lithium ion secondary battery. In this study, BET was examined for use as a new nontoxic diluent for the fabrication of highly porous PE membranes by thermally induced phase separation process. It was confirmed that BET can be used as a new diluent for the fabrication of the PE membranes by exploring upper critical solution temperature type phase behavior of PE mixtures with BET. When the porosity of the membrane prepared from the PE/PO mixture was compared with that prepared from PE/BET mixture, the latter was about 1.8 times higher than the former.