A series of asymmetric polyethersulfone (PES) membranes were fabricated using non-solvent induced phase separation (NIPS) process from the system of water/GBL/PVP/PES at 40 °C. Water, acting as the nonsolvent, was added in different amounts to the dope solution to modify the porous structure and enhance the permeability of the membranes. The fabricated membranes were tested systematically for their morphologies, physical properties, pure water fluxes, and rejection capacities against bovine serum albumin molecules (BSA). The SEM images indicated that the pore size and porosity of the top surface increased with increasing water content in the dope, and when 1.5 phr of water was incorporated, the macrovoids in the cross section disappeared and the pores interconnected to form numerous continuous channels within the PES matrix. This membrane exhibited a pure water flux of 642 L m−2h−1bar−1 and a tensile strength as high as 5.6 N/mm2. The dead-end filtration experiments indicated a very high BSA rejection of ~98% for membranes prepared from dopes containing ≤1.0 phr of water, and a moderately high (85%) rejection for that with a higher dope water content.