The effects of maturation time of casting solution and annealing conditions on reverse osmosis (RO) properties of nylon 4 membranes are studied. The effects of plasma treatment conditions, such as supply power, treatment time, and gases used in reactor, on the water contact angle, surface energy, water flux, and salt rejection of nylon 4 membranes are considered. The best results of 4.86 × 10−5 g/cm2 s of water flux with 74.2% of salt rejection for 1 wt % NaCl aqueous feed solution can be obtained in the conditions of plasma treatment for 80 W–3 min in a reactor with O2 on 25 wt % formic acid–4 h annealing nylon 4 membranes. The hydrophilic monomers, such as 1-vinyl-2-pyrrolidone (VP), 2-hydroxyethyl methacrylate (HEMA), and acrylic acid (AA), are plasma deposited onto the chemical and/or plasma etched nylon 4 membranes to improve the salt rejection without decrease in water flux. The salt rejection of 90.9% and water flux of 3.64 × 10−5 g/cm2 s of the deposited membrane are obtained in the conditions of depositing AA monomer onto the substrate membrane which was previously treated with annealing followed with plasma etching.