Radiotherapy using ceramic microparticles that act as β-emitters after neutron bombardment is attractive as a minimally invasive option for cancer treatment. Yttria (Y₂O₃) microcapsules (20–30 µm in diameter) are capable of cutting off the nutrition supply in cancer cells through an embolization effect. In the present study, Y₂O₃ microcapsules were prepared via precipitation of yttrium hydroxide from a water/oil (W/O) emulsion and a subsequent heat treatment. The emulsion was prepared by dispersing yttrium hydroxide sol in 2-ethyl-1-hexanol. Microcapsules were obtained by an addition of the emulsion in butanol via dehydration and subsequent aggregation of the yttrium hydroxide. The effects of the rotation speed and surfactant concentration on the diameter of the particles were investigated. The diameter of the microcapsules showed a tendency to decrease with increases in rotation speed during emulsion preparation or surfactant concentration in the oil phase. A high yield of the Y₂O₃ microcapsules with a diameter of 20–30 µm were obtained after a heat treatment at the optimized rotation speed and surfactant concentration. The obtained microcapsules showed high chemical durability in a simulated body environment.