Abstract. In this research, UF membrane was prepared by blending polysulfone (PSf) in DMAc with the addition of PEG400 and low concentration of acetone as additives. The effect of PSf/PEG400 and polymer/solvent ratio on the performance of PSf membrane was investigated during two hours of peat water filtration. The results showed that the increase of ratio PSF/PEG400 from 1:1 to 1:2.5 improved the membrane hydrophilicity. It was indicated by a decrease of water contact angle from 62 o to 33 o and increased water content in membrane structure from 65 to 74,5%. In addition, the pure water flux also increased up to 200 times. A stable flux is achieved when the ratio PSf/PEG is 1:2 and resulted in 84% rejection of humic acid. The increase of polymer concentration in the membrane solution reduced the pure water flux but increased the rejection of humic acid. Higher polymer concentration in membrane solution reduced fouling tendency towards the membrane surface. It showed that flux decline was reduced from 60% to 40% when the ratio of polymer/solvent concentration was changed from 40/60 to 50/50.