Abstract: Graft polymerization of acrylic acid (AA) onto polyethersulfone (PES) membranes was studied by simultaneous irradiation method, using electron beam. The change of grafting yield with irradiation dose and atmosphere was investigated. The grafting of AA on the PES membranes was confirmed by FT-IR, and the surface of PES membranes were observed by SEM. The effects of grafting yield on the fluxes of water and BSA permeation were also studied. It is found that grafting yield first increases and then decreases with the increase of irradiation dose in the atmosphere of nitrogen gas and air. Significant difference of the surface morphologies for original PES, grafted PES (grafting yield is about 11%) and fouled membrane is observed. The results show the change of water flux and BSA permeation flux initially increase and then decease simultaneously with grafting yield increasing.