Abstract: There is provided RE-Ba-Cu-O oxide bulk superconductors in which considerably high critical current density is obtained at relatively high temperature. In the present RE-Ba-Cu-O bulk superconductors, RE is a combination of two or more elements selected from La, Nd, Sm, Eu and Gd, at least one of them being La, Nd and Sm and the remainder being Eu or Gd, in which a parent phase thereof comprises a RE.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.y crystal wherein -0.1<x<0.2 and 6.7<y<7.1, and 5 to 50% by volume of a RE.sub.2 Ba.sub.2 CuO.sub.5 fine dispersed phase having partide size of 0.01 to 0.5 .mu.m. Preferably, a total amount of Eu and Gd in the RE site is 40% by weight or less, while a slight amount of Pt may be added. As a result, the critical current density at liquid nitrogen temperature can be improved to 10,000 A/cm.sup.2 or more under a condition where a magnetic field of 3T is impressed parallel to c axis of the crystal.