Polyethyleneimine (PEI) is an inexpensive low-toxic polymer and is used for the preparation of thin film composite (TFC) nanofiltration (NF) membranes. One serious problem with most of the reported high salt rejecting (ca. 90% divalent cation) PEI-based TFC membranes is the low permeate flux and low average monovalent to divalent anion selectivity. There is also no comparative study of antifouling property of the PEI TFC membranes with conventional poly(piperazineamide) TFC membrane. Herein, we report an approach for the in situ manipulation of permeation behavior and antifouling property of PEI -based TFC NF membrane. The approach was based on the use of dextran (Dex) conjugate of PEI (PEI-Dex) as a new monomer or co-monomer for the interfacial polymerization (IP) with trimesoyl chloride (TMC). Particularly, membranes prepared with mixture of PEI-Dex (2.5%, w/v)+ PEI (0.25%, w/v) showed ca. 86 Lm (-2) h(-1) MPa (-1) pure water permeate flux and 84%, 85% and 37% rejections of (NaSO4)-S-2, MgCl2 and NaCI, respectively. On the other hand, membrane prepared with PEI (0.25%, w/v) alone gave much lower permeate flux (37 Lm (-2) h(-1) MPa-1). The membrane prepared with PEI-Dex (3%, w/v) gave permeate flux as high as 163 Lm (-2) h(-1) MPa-1 with ca. 71%, 72% and 10% rejections of Na2SO4, MgCl2 and NaCI respectively under similar experimental conditions. All the PEI -based membranes prepared under suitable experimental conditions showed high average monovalent to divalent ions (cation and anion) selectivity. The active layer thickness decreased with increasing proportion of added PEI-Dex into the aqueous bath. This is the reason for the enhancement of permeate flux of the PEI-Dex membranes. On the other hand, low membrane surface charge at neutral feed pH is responsible for the higher average monovalent to divalent ions selectivity. The PEI -based membranes exhibited much improved antifouling/ antiscaling properties compared to that of conventional poly(piperazineamide) TFC NF membranes. These types of membranes are suitable for long-term water softening without frequent cleaning step. (C) 2016 Elsevier B.V. All rights reserved.