The stability of Cu precipitates in thermally aged Fe-Cu alloys after high-energy particle irradiation was investigated using a mono-energetic positron beam apparatus. The S-parameters did not change with increasing incident positron energy above 15 keV, which means positron annihilation at the surface does not influence the Doppler broadening (DB) spectra. In a non-aged sample, the S-parameter increased, while the W-parameter did not change upon ion irradiation at above 10 keV. By contrast, in the thermally aged sample, the S-parameter increased, while the W-parameter decreased upon ion irradiation. The DB spectra of thermally aged and non-aged samples have nealy the same shape after high-energy particle irradiation. In both thermally aged and non-aged samples, almost all positrons annihilate with electrons in Cu, and vacancy clusters covered with Cu atoms are formed upon neutron irradiation. This agrees with the results of previous studies. However, the defect growth process may be different in aged and non-aged samples.