Polysulfone (PSf) ultrafiltration (UF) membranes were prepared by non-solvent induced phase separation process using two kinds of additives, i.e., polyvinylpyrrolidone (PVP) as a representation of water soluble polymers and polyaniline (PANI) nanofibers as a representation of nanomaterials. The effects of PVP and PANI nanofibers on membrane formation mechanism, structure and performance were systematically investigated, compared and analyzed. The thermodynamic and rheologic properties of the casting solutions were studied through cloud point titration measurement and viscosity measurement. Variations of the casting film during preevaporation and after immersed into the coagulation bath were observed through mass variation measurement, optical microscopy measurement and light transmission measurement. Membrane structure was characterized by X-ray photoelectron spectroscopy, scanning electron microscope and contact angle measurement. Membrane performance was evaluated by pure water flux, protein rejection, antifouling property, additive stability and mechanical property. The results demonstrated that at the same additive content, PSf/PANI nanocomposite membranes had higher protein rejections, higher antifouling property, better additive stability and larger breaking strength than PSf/ PVP membranes. The correlations among membrane formation mechanism, structure and performance were also discussed in detail. (C) 2011 Elsevier B.V. All rights reserved.