Results of the analyses on crystal structure and chemical composition of dark-green crystals mainly found in an Y-Ba-Cu oxide specimen which shows highc superconductivity are reported. Crystal structure is investigated by the electron diffraction method and is determined to be orthorhombic with lattice parameters =1.22 nm, =0.561 nm and =0.714nm, and with a space group Pnma (No. 62) or Pna21 (No. 33). Expected -values are presented and are successfully compared with the X-ray powder diffraction spectrum obtained from this oxide. Chemical composition is analyzed by the EPMA method and is determined to be Y2BaCuO5 for the oxide.