Implementing the generation IV nuclear reactor concept to nuclear service needed elevated temperature component analysis and design methodologies. These components experience elevated temperature loads such as creep, fatigue, creep-fatigue, and ratcheting. Traditional elastic, simplified inelastic, or elastic-perfectly plastic analysis and design methods per Section III, Division 5 fails to capture the detailed material behavior. Full inelastic analysis methods governed by the unified constitutive model (UCM) have shown promise in capturing the complicated material behavior at elevated temperatures. Current Section III, Division 5 design code do not provide guidelines for UCM based full inelastic analysis of Gen. IV components using finite element methods. Gap analysis of the current Section III, Division 5 design code indicated the need to develop unified constitutive models (UCMs) and their optimized parameter sets for performing full inelastic analysis. This paper presents a UCM with two and four nonlinear kinematic hardening rules for diffusion bonded (DB) Alloy 800H. Experiments on DB Alloy 800H are performed for developing and validating a UCM. The developed UCM captures the critical fatigue, creep, and creep-fatigue response features such as hysteresis loop, stress relaxation, rate effects, and cyclic hardening demonstrated by DB Alloy 800H experiments performed. Description of the UCM model developed and its optimized parameters for DB Alloy 800H and cor-responding simulations are presented. Future works to further develop the UCM are discussed.