Abstract: A low-activation F/M steel 12Cr3WVTa micro-alloyed with V and Ta was designed by Thermo-Calc thermodynamic simulation in combination with experimental methods. Its microstructure and precipitation after water quenching at 1 050℃ and then tempering at 780℃ were examined by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectrometry (EDS). The results show that the microstructure of the steel after quenching and tempering contains mainly tempered martensite with a small amount of δ-ferrite, as well as precipitates M₂₃C₆ and MX (M=V, Ta; X=C, N). M₂₃C₆ carbides distribute mainly along the lath boundaries and the phase interfaces, while MX precipitates within the tempered martensite and the δ-ferrite. The steel has good tensile properties at room temperature and high temperature (600℃), and its tensile strength and yield strength are 507 MPa and 402 MPa at 600℃, respectively, which meets the tensile property requirements of cladding tubes for supercritical water cooled reactors (SCWR).