In this study, cost-benefit analyses based on life cycle assessment is applied to optimize the recycling of processing water for semiconductor factories. A representative 8-inch semiconductor wafer manufacturing plant is selected and seven existing or potential processing water-recycling sources were set, reverse osmosis (RO) reject, ultrafilter (UF) reject, multimedia filter (MMF), on-line analyzer drain, cation/anion (C/A) filter and merry-go-round (MGR) filter backwash water (including C/A sensor drain), wafer process organic drain and wafer process inorganic drain, marked as point 1 to 7, respectively. To sort the water-recycling sources in ascending order of the results of life cycle cost analyses, they were point 4, 5, point 2, 3, point 1, 7 and point 6, with life cycle the cost about 100,000$, 350,000$, 1000,000$ and 2000,000$, respectively. The order changed when they were sorted by their unit recycling-water costs; that was point 1, 2, point 5, point 3, 4 and point 7, with the unit recycling-water costs 0.2$/ton, 0.3$/ton, 0.4$/ton and 0.5$/ton, respectively. The analyses also evaluated the water recycling practice for various assumed unit tap water price. The optimal processing water recycling strategies based were proposed and corresponding optimal water-recycling rates were 24%, 64%, 81%, and 85% for water price 0.373$/ton, 0.578$/ton, 0.75$/ton and 0.945$/ton, respectively.