Abstract As a part of the activity to establish joining methods to fabricate in-vessel components and to evaluate their performance under fusion environment, microstructural evolution was studied by means of the dual-ion irradiation method. The base material used in this study was solution annealed 316 stainless steel. Welded joints were made by the tungsten inert gas (TIG) welding method and electron beam (EB) welding method. For the prospective improvement of swelling in welded joints, modified TIG or EB welding procedures utilizing titanium or nickel foil insertions were investigated. TEM disk specimens from various positions of welded joints were irradiated to 25 dpa at 673, 773 and 873 K. He/dpa ratio in Ni/He dual-ion irradiation was 15 appm He/dpa. The present results indicate some concern about the microstructures which result in the fusion zone and heat affected zones.