Separation process of proteins was analyzed by Nernst-Planck (NP) based model to determine two parameters, i.e. effective charge of the membrane (φ) and actual membrane porosity (Ak). By considering interaction between membrane and proteins as well as interaction between the protein molecules, those parameters are used to predict the fouling phenomenon of proteins on the charged membrane surface. In this paper, the parameters in N-P model was determined based on experimental data of protein separation (bovine serum albumin/BSA and hemoglobin) by a sulfonated polyethylene membrane at solution pH between 5 to 8. The results show that the most effective separation occurred at pH 5 (α= 5). The actual porosity (Ak) of the membrane is influenced by the type and charge of proteins. The decrease of negative charge along with the increase of solution pH increased the porosity of the membrane, which reduced the rejection of proteins. The φ values are between 0.99996 to 1.0000, which means that the fixed charge on the membrane structure is higher than the concentration of proteins. The φ value was not influenced by the presence of charge solute in solution.