The complete hydrolysis of poly(2-ethyl-2-oxazoline) (PEOX) produces poly(ethylene imide) (PEI). While PEOX and poly(acrylic acid) (PAA) can form hydrogen-bonded polymer complexes that are precipitated from solution at low pH values, PEI and PAA can form polyelectrolyte complexes in the solutions with neutral pH values. In this work, partially hydrolyzed PEOX (PEOX-EI, hydrolysis degree 60%) and PAA are selected to prepare nanofiltration (NF) membranes. In order to obtain homogenous mixed solutions that are suitable for the membrane preparation, alkaline (NaOH) is precisely added into the system to tune the hydrogen bonding and electrostatic force between the PEOX-EI and PAA chains. FT-IR spectroscopy is applied to investigate the interactions between the PEOX-EI and PAA chains. The rheological behaviors of the homogenous mixed solutions of PEOX-EI and PAA are studied in order to optimize the membrane casting conditions. By taking the advantage of the reaction between the imine groups in PEI chains and terephthaloyl chloride, the cast membranes are further cross-linked to enhance the membrane stability. The NF performance of the complex membrane is then evaluated using methyl blue (MYB) solutions, resulting in a MYB rejection rate of 99.2% and a flux of 55.8 L/m(2).h under the pressure of 6 bar.