This paper reports the fatigue analysis for a failure of a shaft in a wind turbine in which reinforced-welding elbow to support the shaft load. The shaft was broken near the reinforcing elbow. When designing mechanical and structural components, the diameter and the characteristics of materials have been considered not to exceed the permissible limit. However, after 40 days of the cycle operation, the shaft was broken at the accepted loading conditions within reasonable limits at a wind speed of 6-10 m/s. The crack initiate fatigue in the shaft is influenced by many variables that may due to uniaxial, bending, torsion, the shape of loading curve, part size, part finish, operating temperature, and atmosphere. For these reasons, it is important to investigate the fatigue behavior on the failure of the shaft. The main goal is to investigate and analyze the fatigue behavior on a broken shaft of the wind turbine gearbox to determine the features that cause the failure. A microscopic investigation of the fatigue-fracture surface was done. The microscopic analysis was carried out in terms of the chemical composition using ARL Spark Spectrometer, energy dispersive spectroscopy (EDS), the morphology of the fatigue by using scanning electron microscopy (SEM) and hardness test. Moreover, a theoretical analysis was made to illustrate the results and comparison between these results and the corresponding stress distribution data from a finite element method (FEM) simulation to know how the fatigue fracture began and propagated along the shaft. The result of the analysis shows that the fatigue occurred due to the unbalanced of the bending and torsion stress in the junction of the reinforced-welding elbow with hardness testing at the extreme cracking area is 435 HV. It is emphasized that the microstructure of fatigue surface structure shows sensitization to facilitate intergranular cracking and fatigue corrosion.