Positron annihilation spectroscopy is a nondestructive technique and a powerful tool for testing defect clusters,especially vacancy-type defects in condensed matter.Positrons are produced naturally by β+ radioactive decay associated with gamma ray emitting.In the dominant decay mode of a thermal positron and electron,two gamma rays are emitted.We can obtain the information of the size and the density of defects from positron annihilation lifetime measurement.In addition,in the laboratory frame,the energies of the two photons emitted by the annihilation of a positron and electron are different,which is proportional to the longitudinal component of the electron-positron momentum in the direction of gamma emission.Through the measurement of photon energies,information about momentum distribution of core electrons can be obtained.In the present presentation,the formation of Cu precipitates and point defect clusters in Fe-Cu alloys will be introduced.Cu precipitates obstruct dislocation motion during deformation,which increases the hardness and decreases the ductility of the alloy,thus inducing embrittlement in Fe-based alloys containing Cu impurities,such as old commercial reactor pressure vessel steels.In addition,phase separation of Fe and Cr in Fe-Cr alloys,which are expected to be used for the first-wall component of the fusion reactors as well as for the Generation Ⅳ nuclear fission system,is also diccussed.