Radiation-induced grafting of styrene into polyethersulfone (PES) films were investigated by simultaneous method in solution using gamma-ray from a radio nuclide Co-60 source. The starting materials were prepared via casting method of the polymeric solution. The grafted membranes have side chains of polystyrene which may be sulfonated to become proton conductive to be used as polymer electrolyte membrane in fuel cell. The localized energy of high intensity gamma-ray induced breaking of chemical bonds leads to free radical formation, a reactive species having a single unpaired electron. The radical start a conventional polymerization sequence comparable with that obtained with a chemical catalyst acting as initiator. The effects of grafting conditions include monomer concentration, irradiation total dose, dose rate and addition of cross linking agent, were studied by means of the techniques of structure analysis, morphology, thermal degradation and crystallinity. The degree of grafting (DOG) was analyzed by percentage of the weight increase. As a result, the reaction always follows the same pattern: DOG increases rapidly initially whilst propagation is the main reaction, then more slowly as termination becomes more frequent.