Melt textured growth YBCO superconductors were fabricated by the top seeding method using Sm1.8 (Sm/sub 1.8/Ba/sub 2.4/Cu/sub 3.4/O/sub 7-x/) seed. The relationship between the Y211 particles and the platelet structures was investigated by micro-structural analysis using SEM. The microstructures of melt textured YBCO superconductors have been examined by TEM and HRTEM. The results of TEM studies clarified the direction of crystal growth and a variety of micro-defects such as twin structures and stacking faults that might behave as pinning centers. Our studies were focused on the stacking faults among those micro-defects by HRTEM and formation mechanism of the stacking faults was studied. The stacking faults formed during the tetragonal to orthorhombic transition that occurred at 450/spl deg/C in oxygen annealing. The platelet structures were clearly observed by SEM due to the chemical etching effects. The lengths of stacking faults were increased as the oxygen annealing time increased from 1 hr to 50 hr. The stacking faults were considered to relate to the oxygen contents, as the platelet structures were. The results suggested an oxygen diffusion model for the formation of the stacking faults.