As-annealed Fe–1.5 wt%Cu alloys were deformed by cold rolling. Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectra (DBS) were used to analyze the microdefects in the alloy induced by deformation. Cu precipitates were formed uniformly in the alloy after annealing at 1173 K. To investigate the effects of Cu precipitates on the formation and migration of defects, well-annealed pure Fe samples were prepared as a reference. PALS results show that the value of positron lifetime (both long lifetime τ2 and mean lifetime τm) in the alloy is smaller than in pure Fe during deformation. This indicates that Cu precipitates restrain the growth of vacancy clusters. Results of DBS show that the S parameter in pure Fe is larger than in the alloy, and more deformation is needed for the S parameter to be saturated in the alloy.