Ultrafine Cu precipitates in neutron-irradiated Fe-0.3wt%Cu model alloys are studied by positron lifetime and coincidence Doppler broadening of positron annihilation radiation. We found that irradiation-induced vacancies bound with Cu solute atoms make aggregations to form vacancy clusters (microvoids) and the inner surfaces of the microvoids are mostly coated with Cu. atoms. The microvoids recover by post-irradiation annealing up to 400 degreesC. However, the Cu aggregations survive up to higher temperatures as defect-free ultrafine Cu precipitates.