Abstract. This paper presents results of conceptual design activities and supporting R&D's of a solid breeder blanket system for the demonstration of power generation fusion reactor (DEMO blanket), which is cooled by supercritical water. The Fusion Council of Japan developed the long-term research and development program of the blanket in 1999. Among the program, Japan Atomic Energy Research Institute has been assigned as a hub institute for developing a solid breeder blanket system in Japan. To make the fusion DEMO reactor more attractive, higher electrical power efficiency of more than 40 % has strongly been envisaged. The design work has shown the feasibility of the first wall thermo-mechanical performance and tritium breeding performance of the blanket. In parallel with the design activities, engineering R&D's have extensively been conducted, which cover all necessary issues, such as, material development for structural materials, tritium breeding materials and neutron multiplier materials, neutronics experiments and analyses, and development of the fabrication technology of the blanket module.