X-ray measurement is a very useful tool for bearing failure analysis. Compressive residual stress is created under the bearing raceway if bearings are used under a contact stress higher than the critical value. Residual stress distribution measurement of a used bearing enables to presume contact stress in service. Half height breadth of diffracted X-rey also changes with the fatigue of the material. Therefore X-ray measurement may estimate the degree of fatigue of a used bearing. Fracture surface of a fractured bearing has sometimes a high tensile residual stress at the surface, from which we can presume the fracture toughness of the material and then calculate the fracture stress.X-RAY diffraction is a very useful tool for bearing failure analysis, because X-ray diffraction technique can detect sensitively macro and microscopic lattice changes of a surface layer of crystalline materials. We have been applying X-ray measurement for failure analysis of used bearings for over ten years. In this paper, estimations of contact stress, remaining life of used bearings, or fracture stress of fractured bearings are described as examples of X-ray failure analysis.