High water flux polyamide thin film composite (TFC) forward osmosis (FO) membranes with porous supporting substrate layers made from polysulfone (PSf) and polyethersulfone (PES) blends of varied PSf/PES ratio were prepared. The impact of PSf/PES ratio on the porous structure of the supporting substrates and resulting properties of TFC-FO membranes formed were investigated. There were three distinguished sections observed in the cross-sections of the substrates, a top dense skin section, a central porous section, and a bottom open section according to SEM images. With the joint favoring effects from maximum open bottom section percentage in the cross-section and maximum surface roughness, membrane samples made from substrates with a PSf/PES ratio of 2/3 exhibit a highest water flux of 27.6 L/m(2) h for 2 M NaCl draw solution from a de-ionized water feed solution. And their water flux reached 12.7 L/m(2) h under PRO mode and 10.6 L/m(2) h under FO mode from an ocean seawater feed solution containing 3.5 wt.% of NaCl. This high flux is related to the existence of heavily populated cone shaped protrusions observed on the surfaces of formed TFC-FO membranes, resulting from amplification in initial surface roughness of the porous substrate during the active layer forming interfacial co-polymerization process. (C) 2014 Elsevier B.V. All rights reserved.