Abstract: Globularization of lamellar α phase was stated by the grain boundary separating model. It was elucidated that globularization of secondary (lamellar) αphase in the transformed microstructure is the main flow softening mechanism of TA15 titanium alloy. A unified viscoplastic constitutive equation of TA15 titanium alloy was developed based on the globularization mechanism. Some physical variables including globularization of secondary (lamellar) αphase, normalized dislocation density, isotropic hardening, plastic deformation induced temperature rise, and phase transformation during deformation were comprehensively considered in the constitutive model. Material constants in the constitutive model were determined and optimized by a genetic algorithm. The constitutive model can accurately predict the flow stress of TA15 titanium alloy in hot deformation.