[en] High T_c superconductivity has been observed both resistively and magnetically in the new U(Th)-Y-Ba-Cu-O systems. The zero resistance temperature of the three samples with nominal composition of U_0.1 Y_1.1 Ba_0.8 Cu O_4-z, U_0.15 Y_1.05 Ba_0.8 Cu_4-z and Th_0.3 Y_0.8 Ba_0.8 Cu_4-z is 87K, 79K and 74K, respectively. The result of ac magnetic susceptibility measurement implies that the superconducting state is realized in the U-doped samples below 90K. The stoichimetry of the U-Y-Ba-Cu_O superconductors has the aid of Rutherford backscattering and nuclear reaction ¹⁶O(d, p)¹⁷O. The analytical results show that the 'real composition' of somples may be different from that of the nomial composition. The X ray diffraction analysis of the U-Y-Ba-Cu-O system shows that none of the peaks can be fitted to the uranium oxides and the other raw materiale structure, and its pattern may be attributed to new multiple phases with structure different from the known singlephase superconducting oxides