The effect of thermal exposure on room temperature tensile ductility of cast TiAl alloy with directional lamellar microstructure was evaluated at 750 ℃ for 48~300 h in atmosphere. By preloading, unloading, dye-penetrating followed by reloading until fracture for exposed samples, initiation and propagation behavior of the microcrack triggered by surface brittle layer was mainly analyzed in order to explain that the directional lamellar structure retains a better ductility at room temperature after thermal exposure. The results show that room temperature tensile ductility is still retained above 2.0% and 1.0% after exposure for 150 and 300 h at 750 ℃, respectively. The embrittlement of the directional lamellar microstructure caused by thermal exposure is much less than that of duplex microstructure and the other lamellar microstructures. At a stress of 430 MPa, the microcrack forms at the Al-depleted brittle layer and propagates into the substrate during subsequent loading. Just as the sharp notch, the microcrack can constrain the plastic deformation, which is the main mechanism of the brittlement for TiAl alloy by thermal exposure. The directional lamellar microstructure with the lamellae interface parallel to the substrate surface is obtained, which is good for restraining the micro-crack propagation into the substrate and retaining higher ductility at room temperature after thermal exposure.