In order to investigate the behavior of copper impurity in radiation embrittlement of LWR pressure vessel steels, positron beam Doppler broadening technique was applied to Fe-Cu irradiated by Fe ions. Positron beam Doppler broadening spectra clearly showed production of trapping sites for positrons in thermally-aged Fe-0.9Cu, and the sites are very likely to consist of copper atoms according to the contribution of annihilation with core electrons. In ion-irradiated specimens, copper atoms are likely to be enriched near nanoclusters.