Abstract: In order to study the fracture toughness of nanocrystalline, a material model was proposed. The model contains two types of grains: the ultrafine grains (ranging from 2 nm to 4 nm) and the normal nanograins (ranging from 20 nm to 100 nm). The ultrafine grains could be treated as a part of triple junctions, denoted as super triple junctions, and evenly distributed in the conventional nanocrystalline matrix. In the framework of our model, stress concentration near crack tip initiates intergrain sliding that leads to the generation of edge dislocations at super triple junctions. The effect of dislocation at a super triple junction on the critical intensity factors was studied. The results show that the existence of the ultrafine grains effectively enhances the fracture toughness of nanocrystalline.