A series of YBa2Cu3-xFexOy(x=0—0.5)samples were prepared in air by the standard solid-state-reaction method.The crystal structures and physical properties have been studied by means of positron annihilation technology (PAT), scan electronic microscope (SEM) and X-ray diffraction (XRD).Oxygen contents of the samples were determined by a volumetric method.The results of PAT show that the short lifetime component τ1 and long lifetime component τ2 are all changed anomalously between x=0.12 and 0.15 where the compounds undergo the orthorhombic-tetragonal (O-T) phase transition.In addition,the grain size of the sample with x=0.15 is much larger than that of samples with x0.15,the doped Fe atoms and incoming oxygens tend to associate into clusters rather than to randomly distribute over the crystal lattice.The cluster behavior is in agreement with the results of theory and other experimental results.