Abstract <p>In the paper, we study the influence of high-temperature low-cycle fatigue on the microstructure of a new low-nitrogen and high-boron 10% Cr steel additionally alloyed with cobalt, tungsten, molybdenum, and rhenium. After heat treatment, the lath structure of tempered troostite with a high dislocation density both within laths and at the boundaries of martensite laths is stabilized by particles of grain-boundary carbides M₂₃C₆ and M₆C, as well as by carbonitrides NbX uniformly distributed throughout the matrix volume. The average width of martensite laths was 380 nm, and the density of free dislocations within the laths was 1.4 × 10¹⁴ m^–2. As the strain amplitude increases from 0.2 to 1% during low-cycle fatigue, the number of cycles to failure decreases by ~2 orders of magnitude, while the contribution of the plastic strain component increases significantly. Maximum softening (24%) is observed at the temperature 650°C and strain amplitude 0.6% in the middle of cyclic loading. After low-cycle fatigue tests, the studied steel contains small recrystallized grains free of lattice distortions. Moreover, the lath structure begins to transform into a subgrain structure, with the lath width and subgrain size being dependent on the strain amplitude. The density of free dislocations is hardly affected by the increase in the strain amplitude compared to the initial state, while the density of dislocations at the lath boundaries decreases significantly with strain amplitude, which is due to shortening of the martensite lath boundaries. Fractography shows that oxide particles act as sources of crack initiation at both temperatures of low-cycle fatigue tests.</p>