The mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy was investigated via experiments and molecular dynamics simulations. According to the experimental results, laths occurring in γ-phase grains were identified as γ-phase twins, and increasing strain promoted the γ → α2 (fct → hcp) phase transformation when the alloy was deformed at 1150 °C. A large number of ultrafine γ lamellae precipitated when deformed at 1200 °C, which was attributed to the thermal-mechanical coupling. The atomic mechanisms of the α2(α) → γlamella phase transformations were investigated using molecular dynamics simulations. The results showed that the generalised stacking fault energy along different directions (<1‾010>) exhibited remarkable anisotropy on the basal plane (0001) of the hcp crystal structure, regardless of the ordering or disordering phase. The disordering of the hcp crystal structure significantly affected the formation of the γ lamellae. Regardless of the ideal stoichiometric or off-stoichiometric composition, the stress-induced hcp → fcc crystal structure transformation occurred more easily in the disordered phases. Grain-boundary nucleation dominated the precipitation of the ultrafine γ lamellae. In addition, intragranular nucleation of γ lamellae was observed, which was mainly attributed to the activation and movement of the Shockley partial dislocation loop with a Burgers vector of 1/6<1‾010> whose formation was not directly related to the dissociation of the superpartial dislocation 1/6<112‾0>.