Phase decomposition of some dual phase (α+ γ) Fe-Al-Mn alloys by aging treatment has been studied. α-phase in the dual phase structure decomposed more actively by aging treatment than γ-phase. Initial α-phase has been decomposed into manganese rich β-Mn and manganese depleted new α-phase(α) at the early stage of aging. At the later stage of aging, k-phase of Perovskite structure has been produced additionally. Thereby, the αphase decomposition of the dual phase structure can be summarized as α→β-Mn + α, →β-Mn + α+ κ. γ-phase has been shrinked by aging treatment and disappeared finally. The decomposition of γ-phase is as follows ; γ-r + α→α_0 +β-Mn + κ. All the above results are from the Fe-Al-Mn alloys containing more than 0.2 wt% of carbon. However, the alloy containing 0.04 wt% of carbon did not produce κ-phase with all the increased aging time of 500 hours.