Ti 17 titanium alloy may be classified as a “beta rich” alpha beta alloy. The structure state of billet, hot process history, thermal treatment process and parameters of process would greatly affect the final microstructure and mechanical properties of isothermal forgings. The effects of the interaction between deformation and phase change on the microstructure and mechanical properties for the isothermal forgings of Ti 17 titanium alloy have been studied in this paper. It is indicated that the α plates nucleate uniformly within the matrix and line up in herringbone patterns when the temperature of isothermal forging is within the region of β phase. The strength and plasticity at room temperature and 400℃ is lower if using the rolling bar with coarse grain and the deformation and phase change interacted for few times. The fine and uniform lamellar α structure can been got after an isothermal forging and interacted between deformation and phase change at β phase region and α+β phase region for more than 2 times, then and isothermal forged at β phase region. The more the times of interaction of deformation and phase, the thinner and the more twisted the α pieces. We can get the fine and uniform and globular α structure when forged for more times and just the netted α structure when forged for few times. But, the specification of the strength and plasticity can be meet at room temperature and 400℃ in both cases.