The fracture mechanism of fully lamellar Ti 49at% Al alloy has been investigated by studying the interactions between crack and lamellae or grain boundary. The results indicate that the nucleation and propagation mechanism of crack depend on not only lamellar orientation in grain but also the type of grain boundary. As the angle between tensile axis and lamellae is relatively large, main cracks propagate by nucleation, growth and linkage with interfacial microcracks. As the tensile axis is nearly parallel to the lamellae, translamellar cracks propagate by nucleation, growth and linkage with two types of microcracks, e g interface delamination and translamellar microcracks. Different types of grain boundary have different toughening effects on TiAl alloy, that is, interlock grain boundary nearly parallel to the tensile axis is beneficial to fracture toughness, and the grain boundary nearly perpendicular to the tensile axis is not beneficial.