This article is concerned with the high temperature low cycle fatigue behaviour of a new nano-strengthened martensitic-ferritic steel, MarBN. A range of strain-controlled, low cycle fatigue tests are presented on MarBN at 600 ℃ and 650 ℃, and compared with previously published data for a current state-of-the-art material, P91 steel, including microstructural analysis of the fracture mechanisms. A modified Chaboche damage law, incorporating Coffin–Manson life prediction, is implemented within a hyperbolic sine unified cyclic viscoplastic constitutive model. Calibration and validation of the model with respect to the effects of strain-rate and strain-range is performed based on an optimisation procedure for identification of the material parameters. The cyclic viscoplasticity model with damage successfully predicts fatigue damage evolution and life in the cyclically softening materials, MarBN and P91.