This paper presents a new approach for modeling the deformation response of metallic materials under thermomechanical fatigue loading conditions, based on the evolution of thermal activation energy. In its physical essence, inelastic deformation at high temperatures is a thermally activated process. The thermal activation energy, which controls the time and temperature dependent deformation behavior of the material, generally evolves with the deformation state (γ_p) of the material, in response to the applied stress τ. In the present approach, the inelastic flow equation is integrated for a deformation range where strain hardening is predominant. The simplified integration version of the model only needs to be characterized/validated by isothermal tensile and fatigue testing, and it offers an explicit description of the TMF behavior in terms of physically defined variables. By identifying the dependence of these variables on the cyclic microstructure, the model may also offer a mechanistic approach for fatigue life prediction.