[en] The microstructure of Y-Ba-Cu-O compound, sintered at 950⁰C for 16h, was examined by using transmission electron microscope (TEM). For the furnace cooling sample, two variants of orthorhombic YBa/sub 2/Cu/sub 3/O/sub 7-x/ (O-Y123) twins, grown on (110) and (110 planes, respectively, are crystallographically related to the matrix and produce a Widmanstatten morphology. The orientation relationships between the lath twin and the matrix is (001)/sub T///(001)/sub M/ and [010]/sub M/. The O-123 compound is unstable under the electron irradiation. Twin boundaries are identified as δ-boundaries. The minor phases as Y/sub 2/BaCuO/sub 5/(Y211) and BaCuO/sub 2/ were also observed. For the air cooling sample, no superconductivity above 77K was found. The major phase is tetragonal Y123 (T-Y123), while a small amount of O-Y123 is also existent