The structural and thermoelectric properties of rare-earth (Sm)-substituted RuddlesdenPopper-type Sr 3 Fe 2 O 7 , Sr 3¹x Sm x Fe 2 O 7 (SSFO, x = 0, 0.1, 0.15), were investigated by measuring the temperature dependence of its crystallographic parameters, electrical conductivity, and Seebeck coefficient (S). A sharp increase in electrical conductivity (·) up to 60 S/cm (log · = 1.78 S/cm) at 400500°C occurred as a result of oxygen reduction in the x = 0 sample. S values changed from negative to positive with increasing temperature, presumably due to the increase in the number of oxygen vacancies, resulting in Fe ion hopping conduction. The introduction of Sm increased both S and ·, and the power factor of the oxide prepared with x = 0.1 was approximately fivefold higher than that of the oxide with x = 0.