The elimination of the B2 phase in a β-solidifying high Nb-containing TiAl alloy with β/B2 and γ phases was investigated using different heat treatments, with a focus on understanding the phase transformations and lamellae formation during the process. The phase transformation and lamellae formation during B2 phase elimination differs from that observed in conventional TiAl alloys. During the holding stage of heat treatment, the β/B2 phase is replaced by the α phase through primary phase transformations of β→α and γ→α. Lamellae formation occurs within both α and γ grains during cooling, initiating 30−40 °C below the annealing temperature. This lamellar structure was formed via two main mechanisms: nucleation at grain boundaries followed by growth into the grain, and direct precipitation and growth within the grain. The orientation relationship between the γ phase and its adjacent α phase is (111)_γ//(0001)_α and , with a coherency between the phases characterized by a misfit of approximately 1.7%.