The heat affected zone of a welded STS304 steel has been irradiated in an 1300 keV high voltage electron microscope at 673K and up to 5.4 dpa (displacements per atom) to study the effect of electron-beam irradiation on irradiation damage microstructure. The dislocation loop density of initial irradiation state was increased with electron-beam irradiation. Void size, void number density and void swelling percentage were increased and then saturated gradually with irradiation dose. The depletion of Cr and the enrichment of Ni were also recognized in HAZ part of welded STS304 steel at the grain boundary by EDS analysis, as reported already the same tendency for matrix material.