Abstract: The thermodynamic character of phase transformation of T titanium alloy electron beam welded joint was described. In the point of thermodynamic view, the formation mechanism of different microstructures of T titanium alloy joints in the different heat treatment conditions was analyzed, which provides theoretic base for the method to control phase transformation of T titanium alloy electron beam welded joint by changing heat treatment conditions. The driving force of phase transformation of T titanium alloy electron beam welded joint comes from the difference of chemical free energy between the new phase martensite and the parent phase. The martensite forms throughout the grain, and its orientation was a certain angle. In the case of the post-weld heat treatment which temperature is lower than M_S points, martensite only grows up, but in the case of the post-weld heat treatment which temperature is higher than M_S point, not only martensite grows, but also part of the β-phase becomes saturated solid solution and leaves among the martensite film layers.