Superconducting ultrafine particles of YBCO were prepared by a wet-type jet mill in a high pressure how of methyl-ethyl-ketone (CH3COCH2CH3). As a result, when the mill is operated at a pressure of 1300 kg cm(-2) ultrafine particles of YBCO show oriented polycrystal structures with a particle size 80-120 Angstrom in diameter and with the Lattice parameter of 3.9 Angstrom in the [001] orientation from transmission electron microscopy studies. The particles show the T-c to be at 93 K (onset) after annealing at 935-950 degrees C for 12 h in oxygen.