High-density polyethylene (HDPE) hollow fiber membranes were prepared via thermally induced phase separation (TIPS). Two kinds of diluents such as diisodecyl phthalate (DIDP) and liquid paraffin (LP) were used in the preparation of membrane. In the case of DIDP, liquid-liquid phase separation occurred, while only the polymer crystallization occurred in the case of LP. In all cases, asymmetric structures with the smaller pores at the outer surface were obtained. Effects of polymer molecular weight, air gap distance, water bath temperature and kinds of diluents on the pore size and the water permeability were investigated. The use of the polymer with the higher molecular weight, the shorter air gap distance and the higher bath temperature were effective to obtain the higher water permeability. The membrane prepared with LP showed the lower water permeability than that with DIDP. (C) 2003 Elsevier B.V. All rights reserved.