A series of Ag-doped Tl2Ba2Ca2Cu 3Oy superconducting oxides were prepared by a solid-state reaction and the influence of Ag2O content, with a varying molar ratio which ranged from 0 to 1.0, was also investigated. The normal-state resistance of the samples decreased with an increase in the content of Ag 2O additive, while the critical superconducting temperatures of the Ag-doped samples increased once the Ag2O content was increased. The doping of the silver oxide was indicated from the results of the x-ray diffraction pattern to be favorable to the formation of the three CuO layers with a high-Tc phase Tl2Ba2Ca 2Cu3Oy. The diamagnetic property of the samples was also confirmed by the magnetic measurements to become enhanced by doping with silver oxide. The platelike grains of Tl2 Ba 2CaCu2Oy phase were observed from scanning electron micrographs to be primarily present in the undoped sample, while the amount of the needlelike Tl2Ba2Ca2Cu 3Oy grain increased with an increase in the doping amount of Ag2O.