Abstract: TiAl based alloys were tested in tensile fatigue experiment, and their facture morphology was observed to study the facture mechanism of tensile fatigue. The results show that there were orientations of river pattern and sole crack initiation origin on the fracture of the alloy. The fracture surface was composed of a lot of inter-lamellas which were dominant and little trans-lamellas. The crack initiation origin was located at the corner of fracture surface, and the cracks propagated along different orientations to the edges of the specimen, then the specimen fractured. The fracture mechanism is described as following: several interlamellar cracks successively initiate under tensile fatigue loading at a highly stressed site, then the fatigue crack extends continuously by the cycling of fatigue stress. When the length of fatigue crack reaches the critical crack length matching to the loading stress, which means the Griffiths condition, the cleavage fracture of specimen will take place.