The US/Japan collaboration (Japan-US Program of Irradiation Tests for Fusion Research: JUPITER) has been effective in accumulating an irradiation database and in understanding the mechanism of irradiation effects of reduced activation ferritic steels (RAFS). The irradiation data obtained up to now indicates rather high feasibility of ferritic steel for application to fusion reactors, because of their high resistance to degradation of material performance by both the displacement damage and helium. The martensitic structure of the RAFS consists of a kind of lattice defects before the irradiation, such as dislocations, lath boundaries, precipitates and carbides, which strongly reinforce the resistance to displacement damages through absorption and annihilation of the point defects generated by the irradiation. Transmutation helium can be trapped at those defects in the martensitic structure so that the formation of helium clusters at grain boundaries, which causes intergranular embrittlement, is suppressed. The martensitic structure of the RAFS is considered to be appropriate for fusion structural material. Efforts to increase high temperature strength have been made for RAFS.