Mechanical damages and their formation mechanisms in SOFC were investigated. Generalized schemes of the occurrence of mechanical failures were summarized, and among them, common features in various cell designs were extracted. They were discussed in three categories of (a) deformation of a cell due to its asymmetry, (b) propagation of mechanical defects, and (c) effects of irregular operations. Prototype-cell experiments were designed to reproduce the key features, and to develop a quantitative evaluation scheme of operational margins for prediction of the short and long term reliability of a specific cell design. Numerical calculation tools based on finite element method and homogenization method are designed. Experimental methods were developed to acquire high-temperature mechanical behaviors of the materials and cell stacks.