The present invention provides a process for the preparation of YBa.sub.2 Cu.sub.3 O.sub.7-x superconductor which comprises surrounding a sintered material in which the molar ratio of Y:Ba:Cu is 2:1:1 with liquid-forming powder and subjecting the powder compact to isothermal heat-treatment at a temperature below the peritectic temperature of YBa.sub.2 Cu.sub.3 O.sub.7-x. The YBa.sub.2 Cu.sub.3 O.sub.7-x superconductors prepared according to the present invention have aligned grain structure in one direction and thus exhibit a high critical current density.