Abstract: Ethanol, a coagulation medium, plays an important role in producing high performance asymmetric nanofiltration membrane. Thus, the effect of ethanol composition in coagulation bath on the performance, morphology and properties of Polyethersulfones (PES) flat sheet membrane was studied. The performances of the PES membrane were obtained from the Pure Water Flux (PWF) experiment and salt solution rejection experiment. The results showed that the permeability coefficient (Pm) in the pressure range of 0-10 bar for ET 0 (100% water), ET 50 (50% ethanol) and ET 100 (100% ethanol) were 11.971, 8.464 and 3.0823 (m3 m–2 sec MPa), respectively, which showed that: Pm ET 0 > Pm ET 50 > Pm ET 100. The rejection range follows as ET 0 < ET 50 < ET 100, which were 0-10, 0-18 and 0-20%, respectively. The morphology of membranes were vary from finger-like to sponge-like with the increase of ethanol composition in coagulation bath. This shows that higher ethanol composition may create thick membrane layer and cause the water fluxes to be declined, thus enhancing the membrane rejection. It was found that ET 50 was the optimum ethanol composition for its high rejection with moderate flux and thus suitable for industrial application.