Several high-manganese austenitic steels and ferritic steels were studied by means of mechanical tests in order to develop materials for reduction of radioactivity of the first wall material after irradiation in a fusion reactor. After testing the commercial high manganese austenitic steels (that is, Fe-22%Mn-13%Cr-5%Ni), a new type of high manganese austenitic steels (that is, Fe-17%Mn-6.5%Cr-4.5%Si and Fe-22%Mn-6.5%Cr-4.5%Si-0.2N) was developed. Tensile tests were performed using miniature specimens in the temperature range between 4 and 500 K. Instrumented Charpy impact tests were also performed using quarter-size specimens at 77 to 550 K. It was found that the new steels show high strength and good ductility before irradiation. Neutron irradiations were carried out in the Kyoto University Reactor (KUR) and the Japan Material Testing Reactor (JMTR) reactors. After neutron irradiation up to 1 × 1020 n/cm2, the high-manganese steels and high silicon-manganese steels still showed high strength and good ductility.