In fabrication of an SOFC, dissolution of a small amount of NiO into YSZ electrolyte is often observed. With expectation of changing mechanical properties under similar conditions, measurements were carried out with the samples treated before and after annealing in reducing atmospheres. Elastic moduli were measured by a resonance method, and fracture strength was evaluated by a small punch technique. Some of the sample pieces were annealed with flowing 1%H2-1.2%H2O mixed gas at 900℃ for 15 h, 50 h, and 170 h. Raman scattering spectroscopy revealed that the phase of the annealed samples showed gradual transition from cubic to tetragonal. Young's modulus and fracture stress of the as-sintered sample showed minimum at a temperature around 400 to 500℃ where large internal friction was observed. This suggests that the mechanical properties were determined not by the concentration of Y2O3 but by the crystal structure that mainly appeared in the sample.