Microporous high-density polyethylene (HDPE) and low-density polyethylene (LDPE) hollow fiber membranes were prepared from polyethylene-diisodecyl phthalate solution via thermally induced phase separation. Effect of the polyethylene density on the membrane structure and performance was investigated. The HDPE membrane showed about five times higher water permeability than the LDPE membrane because it had the larger pore and the higher porosity at the outer membrane surface. The formation of the larger pore was owing to both the initial larger structure formed by spinodal decomposition and the suppression of the diluent evaporation from the outer membrane surface due to the higher solution viscosity. (C) 2004 Wiley Periodicals, Inc.