The effect of Mn addition on the phase transformation behavior of a thermally grown FeO scale formed on Fe–Mn alloys was investigated. The FeO layer contained Mn, and the Mn/Fe ratio in FeO was similar to that in the alloy substrate. Only 2 wt% Mn addition in the Fe substrate was found to delay significantly the overall transformation time, which includes magnetite (Fe3O4) seam formation, Fe3O4 precipitation, and eutectoid product formation. Decrease in the area fraction of eutectoid lamellar region in the oxide scale and increase in the lamellar spacing in the eutectoid structure after the phase transformation of a (Fe, Mn)O scale suggested that Mn addition decreases the driving force for the FeO decomposition via eutectoid reaction, i.e., extends the FeO phase region to lower temperature. 2 wt% Mn addition was estimated to decrease the eutectoid temperature of FeO about 40 °C, which agreed with the calculated Fe–Mn–O phase diagram.