The tensile ductility of the Ti-23Al-17Nb (at.%) alloy laser welded joints is very low at 650°C, so the embrittlement of the joints at high temperature was investigated. The metastable coarse B2 phase of the as-welded metal transformed into coarse O phase in the weld zone when the joints were heated up to 650°C. This type of phase transformation was based on shear transformation. Coarse orthorhombic O phase has lower ductility compared with body-centered cubic B2 phase. Meanwhile, the shear transformation from B2 to O phase could induce stress-strain concentrations in the grain boundary, which seriously weakened the grain boundary of the columnar grains and led to the brittle fracture characteristics in the weld zone during tensile test at 650°C.