The　microstructural　evolution　of　two　model　alloys,　Fe-NiAl　and　Fe-NiAl-Mn,　during　aging　at　500　degrees　C　was　investigated　using　atom　probe　tomography　(APT).　Vickers　micro　hardness　measurements　showed　that　the　addition　of　Mn　increased　the　hardness　and　decreased　the　time　to　reach　the　peak　hardness.　Both　alloys　exhibited　a　softening　process　at　the　initial　stage　of　aging.　The　APT　results　showed　that　adding　Mn　increased　the　number　density　of　NiAl　nanoparticles　significantly,　and　Mn　atoms　partitioned　into　the　NiAl　nanoparticles,　occupying　the　Al　sites　preferentially.　The　precipitation　of　NiAl　nanoparticles　was　accelerated　by　the　addition　of　Mn　because　Mn　increased　the　nucleation　driving　force　for　NiAl　precipitates　and　reduced　the　strain　energy　for　the　nucleation.