The electrical conductivity of La 0.6Sr 0.4Co 1-yFe yO 3-δ (y=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) were measured as a function of P(O 2) and temperature. The conductivity variation can be well explained only by change in electron-hole concentration due to oxygen nonstoichiometry regardless of temperature. Conductivity relaxation analyses were also conducted to determine chemical diffusion coefficients by non-linear least squares fitting using the solution of Fick's diffusion equation. Oxygen vacancy diffusion coefficients were calculated using the chemical diffusion coefficients and oxygen nonstoichiometry. It should be emphasized that vacancy diffusion coefficients are not independent of oxygen vacancy concentration at lower temperature but steeply decrease with increase in oxygen vacancy concentration. These phenomena may cause the performance degradation of La 0.6Sr 0.4Co 1-yFe yO 3-δ cathode.