A microstructure of the massively transformed γ-TiAl (γm) phase in a Ti-48at.%Al alloy, which was heat treated in the high-temperature α-Ti (disordered hep) single phase field (1683K), followed by ice water quenching, has been examined using high-resolution electron microscopy. The characteristic features of the microstructure originated from the α→γ massive transformation have been clarified in detail, which are as follows. (1) Extremely thin hep plates (about 0.8–2nm in thickness), which are considered to be a retained α phase, are found to exist in the γ
m phase. (2) Twin boundaries are found to be not flat interfaces, that is, twin interfaces are not on the exact (111) mirror plane. This situation is attributed to the existence of a number of partial dislocations at the twin boundaries. (3) Antiphase relationship between the regions either side of the thin rotated domain wall  is confirmed. The validity of this situation is explained by assuming that the thin rotated domain wall has been grown from a simple antiphase domain boundary. On the basis of these facts, mechanism of the α→γ massive transformation has been discussed.