This paper deals with the preparation of organic-inorganic ultrafiltration (UF) membranes by solution casting followed by phase separation method. The silica (SiO2) particles were added to the cellulose acetate (CA) polymer with the increment of 10 wt.% from 0 to 40% by weight using N,N-dimethyl formamide (DMF) as polar solvent. The prepared organic-inorganic membranes were characterized for UF performance such as compaction, pure water flux, % water content, membrane hydraulic resistance, molecular weight cutoff (MWCO) and pore statistics. MWCO and pore statistics were investigated using protein solutions of different molecular weights. It is observed that by increasing the concentration Of SiO2 in CA polymer, the MWCO, pore radius, surface porosity and pore density has been increased. The mechanical stability of the CA/SiO2 blend membranes increased initially and then declined with the addition of inorganic particle above 10 wt.% to the casting solution. The morphological structure was changed with the addition Of SiO2 particles in the casting solution. Further the permeate flux and % rejection of different molecular weight of proteins were investigated which showed an increased protein permeate flux with the decreased solute rejection. Meanwhile, the effect Of SiO2 content in the CA membranes on fouling-resistant ability was studied using BSA solution. The results indicated that increasing the SiO2 content in the casting solution. the reversible fouling resistance dominated the total fouling resistance thereby improving the fouling resistance ability of the blend membranes. (C) 2008 Elsevier B.V. All rights reserved.