Silica nanoparticles with a controlled size in the range of 10-100 nm were synthesized and their surface was treated with various hydrophilic polymers to produce polysulfone (PSf)/silica nanoparticles composite membranes which exhibited enhanced performance in the ultrafiltration process. PSf composite membranes containing silica nanoparticles grafted with a hydrophilic polymer exhibited a much higher water flux than the PSf membrane. Among the PSf composite membranes containing silica nanoparticles grafted with poly(vinyl alcohol) (PVA-g-silica), poly(1-vinylpyrrolidone) (PVP-g-silica), and poly(1-vinylpyrrolidone-co-acrylonitrile) (P(VP-AN)-g-silica), the solute rejection of the PSf/P(VP-AN)-g-silica membrane was the same as that of the PSf membrane, while that of PSf/PVA-g-silica was significantly lower than that of the PSf membrane. The amount of grafted polymer on the particles decreased with increasing particle size. As a result, the water flux of the membranes increased with decreasing particle size, regardless of the type of grafted polymer. The PSf/P(VP-AN)-g-silica membrane showed the best performance among the membranes examined when the particle content and size were fixed. (C) 2014 Elsevier B.V. All rights reserved.