Microstructure stability in the fully lamellar Ti－45Al－8.5Nb(W, B, Y) alloy were investigated by OM, SEM and TEM after long－term thermal cycling (500 and 1000 cyc) at 900 and 1000℃. The results showed that Al－segregation could not be eliminated completely after the heat treatment. After long－term thermal cycling at 900℃, the discontinuous coarsening was inclined to occur in the Al－segregation region in the alloy. And almost no spheroidized precipitates ofα₂ were observed even after 1000 thermal cycles. After long－term thermal cycling at 1000℃, the massive γ grains were generated in the Al－segregation region. After 500 thermal cycles, the spherodized α₂ precipitates were produced within γ grains which were found at colony boundaries. After 1000 thermal cycles, however, the large equiaxed γ grains containing different orientation of plate－shaped precipitates of theα₂ phase were observed within the lamellar structure or at colony boundaries. After long－term thermal cycling at 1000℃, the plate－shaped or particle－shapedα₂, which is coherent with the γ matrix, precipitates on the {111}_γ plane in the γ grain interior.