Three point bending (3PB) fracture tests and unload tests of lamellar TiAl alloys containing B2 phase were carried out to investigate the toughening mechanisms of the TiAl alloy. Double notched 4PB tests were used to evaluate the critical event of cleavage fracture of this alloy. To observe the morphology of fracture surface and crack path, scanning electron microscope (SEM) and energy spectrum analyses were also carried out. The results reveal the following aspects: (1) The load-displacement curve of 3PB fracture tests presents linear elastic relationship, and B2 phase has no effect on the crack initiation. (2) B2 phase is an obstacle to the crack propagation. The crack is arrested by B2 phase at low applied load and bypasses the B2 phase by skipping over B2 phase or crosses B2 phase at high applied load. (3) The critical event of the cleavage fracture of a notched specimen is a propagation of the nucleated interlamellar crack at the notch root. And trans-lamellar propagation is the key factor affecting the whole process of cleavage fracture. (4) The toughening mechanisms of the current alloy include B2 phase toughening, crack deflection, shear ligament toughening and trans-lamellar toughening