The residual stress, which generated by thermal expansion coefficient mismatch between ceramics and metals, is an important problem on strength in ceramic/metal joints. An interlayer, which is a ductile metal, is inserted between ceramics and metal in order to relax the residual stress. In this study, first of all, the analysis of the residual stress produced in joint-cooling process and 4-point bending tests were carried out. Next, from a viewpoint of experimental and fracture mechanics, the effects of interlayer thickness on joint strength in ceramic/metal joints were discussed considering the superposed stress distribution of the residual stress and the bending stress. In addition, the estimation of joint strength was tried to do from viewpoints of fracture mechanics and probability of strength by considering the superposed stress, size and position of potential defects in the ceramics. From the above-mentioned, it is found that the optimum thickness is 0.2mm in this specimen. Joint strength in various interlayer thicknesses can be estimated as normalized strength of ceramics by arranging joint strength to normalized strength considering the scatter of joint strength and the effective volume.