Recently, the ASME approved a new non mandatory appendix to Section III, Division 5, Subsection HB, Subpart B of the ASME Boiler & Pressure Vessel Code providing guidance on the Division 5 design by inelastic analysis methods for Class A components. This appendix provides guidance on constructing and validating nonlinear constitutive models for use with the Code rules as well as approved material models the designer can use without additional validation. An eventual goal for the new appendix is to provide reference models for all the HBB Class A materials. This paper describes an inelastic constitutive model for Alloy 800H, suitable for use with the design by inelastic analysis methods. The model uses a unified viscoplastic form and a detailed kinematic hardening model to capture the key features of high temperature cyclic behavior in Alloy 800H. At lower temperatures the model switches to a rate independent reponse to better capture the available experimental data. The paper describes the model form, calibration database, and details how the model was calibrated and validated against experiment data. The goal for this model is to incorporate it into the ASME Code, via the newly approved appendix to Subsection HB, Subpart B, to continue to expand the range of prebuilt constitutive models available to designers.