In this paper, we discuss the effects of different oxygen partial pressures on the deformation property and fracture characteristics of representative constituent materials for solid oxide fuel cells (SOFCs). The elastic modulus and fracture strength of 8 mol% yittria stabilized zirconia (8YSZ) and 10 mol% gadolinia doped ceria (10GDC) treated under different oxygen partial pressures were evaluated using the small-punch testing method in this study. The specimens of 8YSZ and 10GDC prepared by a sintering process were treated at 800 °C under an oxygen partial pressure in the range of 0.21 to 10-22 atm for 1 hour. The treated specimens were then fast cooled down to a room temperature, and their mechanical properties were measured under an atmospheric pressure condition by using the small-punch testing method. The experimental results revealed that both the elastic modulus and fracture strength of the IOGDC decreased drastically when the oxygen partial pressure of the treatment was less than 10-15 atm, whereas no significant variation in both the mechanical properties was observed for the 8YSZ. The elastic modulus and fracture strength of IOGDC for the treatment under 10-22 atm was reduced down to 10-20 % of those treated under the atmospheric pressure. SEM observations revealed that the fracture surface of the IOGDC specimens was changed from transgranular mode to intergranular mode when the oxygen partial pressure was reduced, whereas the fracture surface of the 8YSZ specimens was transgranular regardless of the different oxygen partial pressures.