The research and development of commercial fast breeder reactors based on incorporating some complementary proposals into the design of Joyo'' and Monju'' are described. In big reactors, hot-cell type exchange systems must be used for spent fuels which have high decay-heats of from 30 to 50 kW. Bent-fuel must be utilized for attaining high bum-up of the fuel and for obtaining high outiet temperature. Material for a steam generator must be reconsidered, since the accidents with sodiuanwater reaction are still reported around the world. In the scaleup of various machine parts, former thermal stress standards must be revised due to increased thickness. Problems related to the swelling of stainless steel at a neutron flux of about 10/sup 23/ nvt must be studied. The sodium void factor, Doppler factor and others must be correctiy estimated. Research and development on the fiel-melting detection system must be performed. (JA)