For the commercialization of solid oxide fuel cell (SOFC), in addition to the electrical reliability, it is also important to ensure the mechanical reliability of SOFC. Thus, the establishment of a suitable mechanical testing method under various temperatures and oxygen partial pressure conditions is a prerequisite for the development of reliable SOFCs. This paper presents the experimental results of the investigation of elastic modulus and fracture strength on the SOFC components under reducing conditions. An in-situ mechanical testing method was developed in order to investigate the elastic modulus and fracture strength of solid oxide fuel cells components under high oxidizing/reducing environments. The elastic modulus was shown to change under reducing condition, and the behavior was most likely due to non-stoichiometry. It was demonstrated that the in-situ mechanical testing method enabled us to estimate the internal stress in the SOFC cell and stack.