Young's modulus, shear modulus, Poisson's ratio and internal friction of La 0.6Sr 0.4CoO 3-δ (LSC) and La 0.6Sr 0.4Co 0.2Fe 0.8O 3-δ (LSCF6428) were evaluated in the temperature range between 298 to 1173 K, and the oxygen partial pressure, P(O 2), range between 1 × 10 -1 to 1 × 10 -4 bar using the resonance method. The apparent Young's moduli of LSC and LSCF6428 first decreased with increasing temperature, and then drastically increased at intermediate temperatures. P(O 2) dependence of the Young's modulus of LSCF6428 showed different trends depending on temperature. At 873 K, the Young's modulus was almost independent of P(O 2), while it increased with decreasing P(O 2) at 973 K. At 1073 K, it increased down to 5 × 10 -3 bar and then decreased in further lower P(O 2). At 1173 K, it decreased with decreasing P(O 2). Such complicated temperature and P(O 2) dependences are considered as composite phenomena caused by the lattice expansion/shrinkage, the oxygen nonstoichiometry change, and the phase transition.