A low temperature plasma was used to graft a positively charged monomer to initiate polymerization on a hydrophilic polysulfone membrane of negative surface charge. The surface modification was characterized by scanning electron microscopy and x-ray photoelectron spectroscopy. Changes in the membrane surface charge and zeta-potential before and after the modification were determined by measuring the electroosmotic flux across the membrane. The effects of the power, radiation time, and polymerizing reaction time on the modification were examined. Adsorption of positively charged lysozyme on the membrane modified with a positively charged monomer was much reduced due to reduced attractive electrostatic forces between lysozyme and the weaker negative charge of the modified membrane surface. Acrylic acid was also grafted as a monomer on the membrane and it gave an intensified negative surface charge. The adsorption of negatively charged bovine serum albumin (BSA) on the modified membrane was significantly reduced due to an enhanced electrostatic repulsive force between BSA and the acrylic acid modified membrane surface. The results show the important role of electrostatic forces in the interaction between protein molecules and a membrane surface, and that these can be controlled by the membrane synthesis or a surface modification to tailor the membrane to the needs of applications.