The effect of 0.2 mass% Yb/Dy additives on the oxidation resistance of Fe-20Cr alloy at 900℃ in air was studied, and EIS was used to investigate the electrical conductivity of oxide scales formed on the alloys at 900℃. Experimental results indicated that the addition of Yb and Dy to Fe-20Cr promoted the formation of a Cr-rich oxide scale, increased the adhesion of oxide scales, and thus improved its oxidation resistance, with a better effectiveness observed for the element Dy. Electrochemical impedance spectra for the oxidized Fe-20Cr-0.2Dy in neutral Na₂SO₄ solution bubbled with N₂ were composed of double capacitive loops, suggesting a two-layer structure of the films, however, which was not obviously observed from EIS of the oxidized Fe-20Cr and Fe-20Cr-0.2Yb. The lowest electrical conductivity was observed for the scales formed on Fe-20Cr-0.2Dy, indicating a better oxidation resistance, as confirmed by kinetic measurements. The capacitance and resistance obtained can be related to the thickness and defectiveness of oxide scales, so electrochemical impedance spectroscopy was a proper investigating method for the oxidation and corrosion of metal and the results of other analytical techniques were in agreement with the EIS data.