Phase transformations and microstructure evolution of high Nb-TiAl alloy after solution treating were investigated using differential scanning calorimetry (DSC), backscattered electron microscopy (BSEM), X-ray diffraction (XRD) and microhardness tester during aging process from 1000 °C to 1300 °C. The results indicated that the microstructure segregation of high Nb-TiAl alloy after solid solution treatment was not completely eliminated and metastable α₂ phase still existed. Two phase transformations, ordered α₂ → disorder α and tetragonal γ → hexagonal α, took place between 1192 °C and 1331 °C during continuous heating. The microstructure of the alloy was lamellar when the sample was aging treated at 1200 °C, while it was clustered at 1300 °C. The microhardness of high Nb-TiAl alloy decreased firstly and then increased with the time increasing when the alloys were aging treated between 1000 °C and 1100 °C; conversely, the hardness increased firstly and then decreased as time went on when the treating temperature ranged from 1200 °C to 1300 °C.