Abstract: The fracture failure analysis of a large-size 20CrMnTi carburized gear was carried out by metallurgical analysis, chemical composition analysis, hardness test, scanning electron microscope analysis etc. The results show that, the gear fracture occurrs at the root of the teeth with the largest bending stress. Intergranular fracture occurs in carburized layer and transgranular quasi-cleavage fracture occurs in the core. The cause of gear failure is the large size of the 20CrMnTi steel gear.The insufficient hardenability of the gear results in the formation of the upper bainite structure rather than the martensite structure in the center. The discontinuous sheet carbide in the bainitic structure is distributed among ferrite bars, which promotes the crack propagation under the action of stress and makes the gear failure.