The microstructure and mechanical properties of the ferritic-martensitic steel 1Cr13MoNbVB in the initial state and after aging at 300 to 650°C as well as after neutron irradiation in BOR-60 and BN-350 over a wide range of irradiation temperatures (275 to 550°C) and doses (2 to 78 dpa) have been examined. The aging temperatures were 300, 350, 400,450,500,550,600, and 650°C. The time of aging varied from 25 to 10000 h. In 1Cr13Mo2NbVB steel there were shown to be substantial differences in the types of precipitate phase found in the two main microstructural constituents of the steel, ferrite and sorbite. The strength properties of 1Cr13Mo2NbVB steel aged in the temperature range of 300 to 450°C increase compared to the initial as-tempered conditions; after aging at 500 to 550°C there is some reduction of strength properties, and at aging temperatures above 600°C, softening. The largest irradiation produced strengthening and embrittlement of 1Cr13Mo2NbVB steel was found at Tirr ⩽ 350°C. The postirradiation aging data suggests that fine (⩽1 nm) irradiation defects make a major contribution to the low temperature irradiation embrittlement (LTIE) of 1Cr13MoNbVB steel.