A constitutive model using dislocation density rate as an internal state variable has been proposed for hot working of near β titanium alloy in this paper. In the constitutive equations, the effects of solution strengthening and dislocation interaction on flow stress were included. Based on the internal-state-variable based model, the constitutive relationship of a new near β titanium alloy Ti-7Mo-3Al-3Nb-3Cr (Ti-7333) for high temperature deformation was established. And the model parameters were obtained by the genetic algorithm based objective optimization method. Calculating results show that the average relative difference between the calculated and the experimental ?ow stress is about 7.2%. Note that the developed internal-state-variable based constitutive model can efficiently characterize the flow behavior of Ti-7333 alloy.