High-Tc superconducting oxide YBa2Cu3Oy has an orthorhombic structure (a=0. 382, b=0. 389 and c=1. 17nm) with layer sequences of (..BaO CuOx BaO CuO2 Y CuO2 BaO CuOx BaO..) along the c-axis. It usually has twinned microstructure with twin plane {110}, causing associated splitting of some of the diffraction spots. It is well known that these characteristic structures of YBa2Cu3Oy can be affected by partial substitutions of cation sites. For instance, tetragonal structure with a=b is stabilized in the compounds Ba2−xLa1+xCu3Oy and Ba2−xNd1+xCu3Oy . Formations of 90-degree domain structures are also observed in these compounds. Recently, Ono et al. prepared a series of compounds, YBa2−xSrxCu3Oy (x=0 to 1.2), to study the effects of substituting Ba by Sr and found the gradual decrease of Tc with increase of x. In the present study, change of the crystal structures and the microstructures in a series of compounds are examined by means of high-resolution transmission electron microscopy.