The phase boundary,of α_2 precipitation at 600℃ in Ti-AI-La ternary system has been de-termined by TEM examination,which shows three kinds of phase regions,namely thesingle α phase region,the transitional phase region where there is superlattice reflection butno α_2 precipitation,and the α+α_2 region.A point of maximum aluminium content was foundto exist on the α/(α+α_2)boundary and the track of the boundary was explained in terms ofthe internal oxidation of lanthanum and the electron concentration rule for α_2 formation. The phase boundary, of α_2 precipitation at 600 ℃ in Ti-AI-La ternary system has been de-termined by TEM examination, which shows three kinds of phase regions, the single phase region, the transitional phase region where there is superlattice reflection butno α_2 precipitation, and the α + α_2 region. A point of maximum aluminum content was found to exist on the α / (α + α_2) boundary and the track of the boundary was explained in terms of the internal oxidation of lanthanum and the electron concentration rule for α_2 formation.