A dislocation-based model for high temperature cyclic viscoplasticity in 9-12Cr steels is presented. This model incorporates (i) cyclic softening via decrease in overall dislocation density, loss of low angle boundary dislocations and coarsening of the microstructure and (ii) kinematic hardening via precipitate strengthening and dislocation substructure hardening. The effects of the primary micro-structural variables, viz. precipitate radii, dislocation density and martensitic lath width on cyclic viscoplasticity, reveal a size effect of initial precipitate radii and volume fraction, with smaller radii and a higher density of precipitate producing a stronger material. A similar effect is also predicted for initial martensitic lath width at temperatures below 500 degrees C. The model is intended for microstructure sensitive design of high temperature materials and components for next generation power plant technology. (C) 2014 Elsevier B.V. All rights reserved.