In this study, the structural phase transition and decomposition of superconducting ceramic, Y-Ba-Cu-O system was studied. The results of the study are as follows. The high Tc superconductor Y₂Ba₂Cu₃O_(7-x) is made in solid state reaction of O₂atmosphere and the valve of X in Y₂Ba₂Cu₃O_(7-x) is 0.1∼0.2. The transition temperature of tetragonal-orthorhombic phase of Y₂Ba₂Cu₃O_(7-x) appear at 790℃-900℃. In the result of measuring the electrical resistance of sample annealed in O₂atmosphere, the electrical resistance show zero at 92K and the best superconductor, Y₂Ba₂Cu₃O_(7-x)(Y123) can be mabe.