Abstract: The microstructural characteristics, phase composition and transformation of the electron beam welded (EBW)joints of Ti -43Al -9V -0.3Y alloy were investigated by means of the optical microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy disperse spectrum.The results showed that the joint microstructure is mainly composed of B2 phase.The partialα₂ +γtwo-phase lamellar structure and granular γ_m phase are observed.The lanthanon Y exists as YAl₂ phase and refined grains.During the heat treatment and EBW of this alloy, different transformation of the liquid metal will take place duo to the different cooling rate in the cooling period.The lamellar microstructure and Widmanstaten structure will form because of low cooling rate.The faster cooling rate can lead to the formation of the γ_m phase.The ordering transformation only happens at very fast cooling rate.The different casting structures with different properties are obtained finally.