The cyclic softening of a reduced-activation ferritic/martensitic steel, JLF-1, is one of main problems for application. The microstructure of JLF-1 steel after cyclic deformation is analysed by transmission electron microscope and the Vickers hardness is tested. Based on the dislocation theory, the numerical relationship between change in hardness and microstructure of JLF-1 （dislocation density, lath width and dislocation cell size） is given.