[en] Melt-textured YBa₂Cu₃O_7-y (Y123) containing fine particles of Y₂BaCuO₅ (Y211) has been prepared from Y123/Y211 powder that was attrition milled with 1 wt% CeO₂ addition, and the microstructure has been examined. Fine and spherical Y211 particles (less than 1 μm in size) are found to be homogeneously dispersed within the melt-textured Y123 domain. Many dislocations are observed to be formed around the trapped Y211 and the Ba(Ce, Zr)O₃ inclusions, which were formed as a result of CeO₂ addition and ZrO₂ introduction from the jar and ball used for attrition milling. CuO stacking faults were also observed around the trapped Y211; these were initiated at the Y123/Y211 interface and extended into the Y123 matrix. Each stacking fault has a lenticular shape, with a width of a few tens of nanometres and a length of a few hundred nanometres, and the faults developed along the [100] and [010] directions of the Y123. The formation mechanism of the stacking fault was discussed together with the formation of the platelet structure (the elongated Ba-Cu-O phase) on the basis of an oxygenation-induced decomposition of the Y123 phase. It is concluded that the prolonged oxygenation heat treatment producing the tetragonal-to-orthorhombic phase transformation is responsible for the formation of the platelet structure and possibly for the formation of the stacking faults. (author)