Fe-Cu model alloys and pure iron were irradiated at 573 K under two different neutron flux conditions (9.4 × 10-8 dpa/s and 1.1 × 10-9 dpa/s) up to about 0.008 dpa. Although defect clusters were not detected by TEM in any specimen, the results of positron lifetime measurement and mechanical property tests clearly indicated an irradiation rate dependence on the microvoid formation and mechanical properties. In positron lifetime measurements, the size of microvoids under low flux irradiation was larger than under high flux irradiation. But the number density was lower than high flux irradiation. In mechanical property testing, more hardening occurred by low flux irradiation. The irradiation rate dependence increased with copper content. These results suggest that the dependence is caused by the precipitation of copper. As copper migrates mainly by the vacancy mechanism, the size and the density of precipitates increase with increasing vacancy migration distance, which becomes longer with a decreasing of the irradiation rate.