Technical Abstract: Surface modification of flat sheet ultrafiltration membranes, polyethersulfone (PES) was investigated to improve the hydrophilicity of the membrane surface thereby reducing adsorption of the proteins onto the membrane. Grafting of hydrophilic polymers onto UV/ozone treated PES was used to improve the hydrophilicity of the commercial PES membranes. Hydrophilic polymers, i.e., poly(vinyl alcohol) (PVA), polyethylene glycol (PEG), and chitosan were employed to graft onto PES membrane surfaces because of their excellent hydrophilic property. The surfaces of modified PES membranes were characterized by contact angle measurement, FTIR, and AFM. The FTIR spectra indicated that PES membranes were successfully modified by grafting of the hydrophilic polymers. The modified PES membranes showed 20 - 50% reduction in contact angle measurements in comparison with those of the virgin PES membrane. Tapping mode AFM technique was employed to investigate the changes of surface topography, cross section, and root mean square roughness of the modified PES membrane surfaces. The modified PES membranes showed elevated roughness (ranging from 7.0 - 25.7 nm) than that of the virgin PES membrane (2.1 nm). It is concluded that grafting of PVA, PEG, and chitosan onto UV/ozone treated PES membranes increases hydrophilicity and lower protein adsorption by 20 - 60% comparing to the virgin PES membrane. Among three hydrophilic polymers studied, PEG showed the most favorable result in terms of contact angle and protein adsorption.