Ultrafiltration (UP) membrane with enhanced simultaneous permeability and selectivity is highly desired. This work reports that polyethyleneimine (PEI) can be used as an effective additive to achieve the objective by tailoring the morphology of polyethersullone (PES) UF membrane. In order to understand the role of PEI on PES membrane formation, a series of membranes were prepared by adding different amounts of PEI to the casting solution.FESEM results indicate the introduction of PEI can essentially increase the pore density and thickness of skin layer. The results of filtration performance demonstrate that the permeability and selectivity can be improved simultaneously by optimizing the PEI content. When the PEI content was 0.3 wt%, the permeability reached a maximum standing at 359.0 L/m(2) h under 0.1 MPa, which is 35.9 times that of pure PES membrane. Meanwhile, the BSA rejection was at a high point of 96.1%. The optimized membrane outperforms the commercial membrane with analogous characteristics on both permeability and molecular weight cut-off (MWCO). Moreover, the demixing evolution during non-solvent induced phase separation (NIPS) process was in-situ observed by optical microscope and UV-vis spectrophotometer. The effect of PEI on the morphology of PES membrane was elucidated and the membrane formation mechanism was proposed. (C) 2014 Elsevier B.V. All rights reserved.