The transformation of phase structure and microstructure for solution treated Ti-1300 alloy during isothermal aging between 350～700℃ were investigated using optical microscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy analyses. Results show that the fine ellipsoidal ω phase formed in the Ti-1300 alloy at 350℃ firstly, then with the disappearing of ω phase, acicular α phase precipitates formed in the β matrix. The decomposition mechanism of this process was β→ω+β→α+β. The glabol β′ phase separated from the β matrix at 400℃ after 1h. With the increasing of holding time, the β′ phase could be served as the nucleation sites for α phase precipitates, thus leading to acicular α microstructure. And the decomposition mechanism was β→β′+β→α+β. After aging at 500℃~700℃, the decomposition mechanism of metastable β phase became as β→α+β. The nucleation site of the secondary α phase was mainly in the prior β grain boundaries and the internal nucleation of the prior β grain, and the grain boundary α phase with same orientation grew into α cluster gradually. Increasing the aging temperature, the thickness of α plate-like phase increased.