Abstract: Reduced activation ferritic/martensitic (RAFM) steel was fabricated by wire and arc additive manufacturing (WAAM) technology. The microstructural evolution was observed by optical microscope, scanning electron microscope and transmission electron microscope and the mechanical properties were tested by a tensile tester to study the effect of heat treatment on the microstructure and mechanical properties of WAAM RAFM steel. The results showed that the microstructure of as-built RAFM steel consisted of dual phases of ferrite and tempered martensite and the average grain size was about 1.51 μm. After heat treatment, the RAFM steel showed no significant increase in grain size (1.84 μm), and high-density dislocations were retained in the microstructure. In addition, a large number of TiO₂ secondary phase nanoparticles, with the grain size of 5 − 10 nm, have precipitated and dispersed in the matrix after the heat treatment. The tensile strength of WAAM RAFM steel was significantly improved after heat treatment while the elongation after fracture was slightly reduced. The tensile strength of RAFM steel after heat treatment was 1080 MPa at room temperature and even at a temperature of 650 ℃, it can still reach about 285 MPa.