Negatively charged ultrafiltration membranes were prepared from Na-form sulfonated polysulfone (SPS), which was made by sulfonation of polysulfone using SO₃-triethyl phosphate complex. Two kinds of SPS, having charge densities of 1.60 and 0.776meq/g, were obtained. Casting conditions were optimized by varying polymer concentration, evaporation temperature and casting period. This membrane showed less fouling when ovalbumin was negatively charged at high pH. Tubular membranes were used to get reflection coefficient and solute permeability with PEG #600 to #6, 000. Using these values and pure water permeability, pore radius and A_k/ΔX were calculated on the basis of the steric hindrance pore model. The radius was 18.7Å and 24.0Å for the high and the low charge density membrane, Ml and M2, respectively. A_k/ΔX calculated from water and solute permeabilities coincided well for Ml, but not for M2. This may be due to the hydrophobicity of the latter.