Fracture strength and Young's modulus of ceria doped with 10 mol.% Gd and 10, 20 and 30 mol.% Y were investigated by means of small punch testing method under controlled temperature (1073 K) and oxygen partial pressure (log((p(O2)/atm)= -1.1, 17.5, -21.4, -22.0). Under the reducing conditions, the color of fractured doped ceria specimen changed from pale yellow to gray; it is therefore indicated that oxygen nonstoichiometry affects mechanical properties of the doped ceria. Under low oxygen partial pressure, Young's modulus of all specimens decreased monotonically. The fracture strength, by contrast, increased at log(p(O2)/atm) = -17.5, and lower oxygen partial pressures; each yttria doped ceria with different Y content showed different variation trends. The oxygen partial pressure dependence of the fracture strength of doped ceria could be arranged as a function of the reduction expansion of the lattice constant a/a0.