In fission and fusion reactor applications, properties of welds and welded joints are key factors that limit component service conditions including their lifetime. Therefore, the availability of reliable repair welding techniques is strongly required in order to reduce the cost of electricity. One of the most difficult and unique characteristics, which repair welding has to overcome, is the radiation damaged microstructure including nuclear transformed gaseous atoms, such as helium from the (n, α) and hydrogen from the (n, p) reaction. There have been many attempts to investigate weldability of heavily neutron-damaged materials for establishing criteria for the repair welding process. The objective of this research is to clarify the mechanisms of weld cracking, especially for heat affected zone cracking in heavily neutron irradiated stainless steels.