Nanocrystalline Fe73.5Cu1Nb3Si13.5B9alloy prepared by the crystallization of amorphous alloy has been studied by using the positron annihilation technique. Positron parameters, i.e., lifetime &tgr;1, &tgr;2, and line shape parameterSare obtained as a function of the annealing temperature. The results show that there exist two types of defects at the interfaces of nanocrystalline Fe73.5Cu1Nb3Si13.5B9alloy: vacancy‐like and vacancy‐like group microvoids characterized by the lifetime &tgr;1and &tgr;2. The former is in overwhelming majority. The changes of the structural defects corresponding to different stages, structural relaxation, and crystallization are discussed. ©1995 American Institute of Physics.