A novel type of polyelectrolyte complex (PEC)/multi-walled carbon nanotube (MWCNT) hybrid nanofiltration (NF) membranes were prepared via in-situ ionic cross-linking between sodium alginate (SA), polyethyleneimine (PEI), and MWCNTs. Chemical structures and compositions of PECs and MWCNT hybrid PECs (HPECs) were characterized by Fourier transform infrared spectroscopy (FTIR). Morphologies of PEC NF membranes (PEC-NEMs) and HPEC NF membranes (HPEC-NEMs) were characterized by field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), and atomic force microscopy (REM); and their hydrophilicity was characterized by water contact angle (CA). Effects of PEI and MWCNT content on the surface zeta potential and NF performance were investigated thoroughly. It was found that the surface charge character of PEC-NEMs changed from negative to positive with increasing the PEI content. The water flux of HPEC-NEMs dramatically increased from 13.4 to 27.0 L m(-2) h(-1) with increasing the mass ratio of MWCNTs to SA from 0 to 0.050, and the MgCl2 rejection maintained at around 93.5% (testing with 1 g L-1 aqueous MgCl2 solution at 25 degrees C and 0.6 MPa). Moreover, the HPEC-NFM-3 exhibited a good Na+/Mg2+ selectivity. This study provided an effective approach for fabricating positively charged NE membranes with enhanced NE performance, which suits well for water softening. (C) 2015 Elsevier B.V. All rights reserved.