The microstructure of Fe-0.6 wt.% Cu alloys irradiated at 290°C to 0.0055 dpa in the Japan Materials Testing Reactor was examined using a conventional electron microscopy technique to study the precipitation behavior of copper in α-iron, in addition to those thermally aged at 500°C for 1000 h and unalloyed Fe. Hardness measurements were also carried out. Results showed that there were two kinds of copper precipitates in thermally aged alloys. The first one was an ultrathin untwinned precipitate that increased in thickness with aging, decreasing in size and number density. The second was small with twinning, that grew little until peak hardness. Age hardening, thus, was attributed to ultrathin copper precipitate. Irradiation, which induced a dramatic hardening in the alloys, eliminated the ultrathin precipitates while the small-twinned ones survived. Two (111) patterns appeared on a (110) selected area diffraction pattern of the α-iron matrix in the irradiated specimens with one (111) pattern analyzed to be due to (110)Cu3Fe reflections (forbidden reflections in fcc crystals) and the other (111) pattern to (220)Fe3O4 reflections. It was concluded that irradiation induced the formation of short range ordered domains of Cu3Fe-superlattice (L12-type) in Fe-Cu alloys, resulting in the dramatic hardening. The precipitation of FeSi was recognized to be enhanced by irradiations in unalloyed iron.