Broadly speaking, there are three kinds of approaches to investigate the deformation and fracture of materials, that is, the microscopic (metallurgical), macroscopic(mechanical) and theoretical (mathematical) researches. It is necessary, however, to bridge the gap which persists among of them. Many investigations on the form of plastically deformed zone at a crack tip, the distribution of plastic strain in plastic zone and fracture criterion
have been made for the purpose to clarify the mechanism on initiation and propagation of cracks. In this paper, the authors report the studies, from the standpoint of microscopic views, on the crystal deformation at the tip of crac's in notched specimen during fatigue process.