In this research, poly(sulfoxide-amide) (PSA) containing hydrophile groups was synthesized via phosphorylation polycondensation. The synthesized polymer was added to polyethersulfone (PES) casting solution at various concentrations for improving membrane properties. Polyvinylpyrrolidone (PVP. 2 wt%) was added in the casting solution as the pore former. The phase inversion induced by immersion precipitation was employed for preparation of asymmetric membranes. The prepared membranes were characterized by contact angle measurement, scanning electron microscopy (SEM), atomic force microscopy (AFM), mechanical strength evaluation and cross-flow filtration. The contact angle measurements demonstrate that the hydrophilicities of PES membrane are strongly enhanced by increasing the PSA concentration in the casting solution. The surface SEM and AFM images show that addition of synthesized PSA in the casting solution results in nano-structure membranes with small surface pore size. The pure water fluxes through the prepared membranes were increased by adding 5, 10 and 15 wt% of PSA in the casting solution. Moreover, milk water permeation and protein rejection were improved. Furthermore it was found that the addition of PSA in the PES casting solution results in membranes with higher mechanical strength. (C) 2009 Elsevier B.V. All rights reserved.