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 stoichiometry of the U-Y-Ba-Cu-O superconductors has the aid of Rutherford backscattering and nuclear reaction

16

O (

d

p

)

17

O. The analytical results show that the "real composition" of samples may be different from that of the nominal 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.