The adsorption of whey protein onto polyethersulfone (PES) membrane was investigated by static adsorption experiments to understand fouling mechanism and optimize the process condition to minimize the membrane fouling. Adsorption isotherm was applied to calculate the isotherm parameters such as adsorption capacity (K-F) and surface heterogeneity (1/n). The K-F values increased about 3.7 times at pH 3.0, 1.5 times at pH 5.2, and 2.3 times at pH 9.0 compared to that at pH 7.0. The hydrophobic interaction by dissociation of dimer structure of beta-lactoglobulin to monomer structure at acidic and alkaline conditions showed the greatly increasing of the amount of protein adsorption by the protein and membrane interaction which might form the strong and rigid protein layer on the polymeric membrane surface. The addition of salt reduced the protein adsorption on PES membrane because of the interactions between charged protein molecules and salt ions.