Phase transformation kinetics of Ti-1300 alloy after solution treatment were investigated under isothermal conditions in the temperature range of 400 to 700℃ by using dilatometric method. The decomposition kinetic equation of metastable β phase in the alloy was constructed under isothermal conditions by analyzing the experimental data in the theoretical frame of the Johnson-Mehl-Avrami theory. The different values of k and n parameters were obtained in the temperature range of 400 to 420℃ and 500 to 700℃, indicating that the mechanism of the phase transformation was different in different temperature ranges. The decomposition mechanism of metastable β phase is β_m→β′+β→α+β at the temperature of 400 to 420℃, whereas the decomposition mechanism of metastable β phase is βm→α+β at the temperature of 500 to 700℃. When the metastable β phase of Ti-1300 alloy was annealed isothermally at a given temperature, the amount of α phase increased firstly, and then reached a stable value with increasing the holding time. Based on calculation and experimental results, time-temperature-transformation diagrams (TTT diagram) of Ti-1300 alloys were plotted for the metastable β phase decomposition under isothermal conditions from 500 to 700℃. And the nose temperature of the TTT diagram of the alloy is around 600℃ for the Ti-1300 alloys.