Detailed examination was carried out for clarifying the strengthening mechanism of complex precipitation during aging at 100∼600℃ in Fe-1Cu, Fe-3Ni-1Al and Fe-1Cu-3Ni-1Al alloys by the measurements of hardness, impact test, differential thermal analysis and the transmission electron micrography observation. Among the alloys studied Fe-1Cu-3Ni-1Al alloy aged at 500℃ showed the largest hardness increase with time while the impact energy value decreased concurrently. The phases precipitated in the ferrite matrix during aging at 500℃ were found to be ε-Cu and NiAl intermetallic compound. It was also observed that the ε-Cu coarsened rapidly and became randomly oriented large particles tending to be overawed whereas NiAl precipitates grew very slowly and kept coherency with the ferrite matrix showing no overaging for a considerable period of prolonged aging. It was concluded that the major strengthening mechanism during aging at 500℃ in Fe-1Cu-3Ni-1Al was the coherent precipitation of NiAl phase in ferrite matrix. The role of Cu in Fe-1Cu-3Ni-1Al was interpreted as the promotion of NiAl precipitation so that the alloy could reach its maximum hardness more rapidly.