A concept for the design of thermoelectric oxides, which is the method to decrease the thermal conductivity without decreasing the electrical conductivity using the complex oxide system, is proposed. The electrical transport properties and the defect structure of La0.45Nd0.45Sr0.1Fe3-delta, which corresponds to Sr-doped LaFeO3-NdFeO3 solid solution, were investigated to confirm the concept. La0.45Nd0.45Sr0.1FeO3-delta was found to show a p-type behavior between 1073 and 1273 K at an oxygen partial pressure of 10(-4) to 1 atm. The values of hole mobility in La0.45Nd0.45Sr0.1FeO3-delta were found to be in good agreement with those of La0.9Sr0.1Fe3-delta. The heat of transport of the holes was also discussed using the small-polaron model.