PURPOSE: To improve purity, strength and light transmission by hot-pressing specific yttria powder to density the powder and subjecting to HIP (hot isostatic pressing) treatment.

CONSTITUTION: Yttria powder having ≥99.9% purity and 2-10 m2/g BET specific surface area is hot-pressed in vacuum under 100-500 kg/cm2 pressure at 1,300-1,700°C, densified into ≥95% theoretical density and subjected to HIP treatment under ≥500 kg/cm2 at 1,400-1,900°C to give light-transmitting sintered material of yttria which comprises a polycrystalline yttria having ≥99.9% purity and has ≥80% straight-line transmission in 3 mm thickness of specimen in an infrared range at 3-6 μm wavelength, ≤100 μm average crystal particle diameter and ≥120 MPa 4 points flexural strength.