Abstract: 9Cr3W low activation martensitic steel was designed by adding more tungsten and adjusting microalloy elements such as V, Ta, Ti and N. The microstructure and hardening, aging and transformation behavior were investigated. Tensile and charpy impact tests were performed. 9Cr3W steel has greatly improved high temperature instantaneous strength compared with the existing low activation ferritic/martensitic steels such as Eurofer97. Feasibility of the steel was analysed for in-core component and fuel cladding application in supercritical water cooled reactor. 9Cr3W steel has advantages in tensile properties at elevated temperature relative to zircaloy and T91 steel and exhibits a lower ductile-brittle transition temperature and a higher upper shelf energy than T91 steel, indicating a superior impact toughness. However, instantaneous strength of 9Cr3W steel at elevated temperature is not high as AL316 austenite stainless steel for the use of in-core component and fuel cladding application in supercritical water cooled reactor.