Undoped and Tb³⁺-doped HfO₂ spherical particles were prepared by a sol-gel method, by using hafnium tetra-t-butoxide (Hf(O-t-C₄H₉)₄) as a precursor of hafnium dioxide, SPAN85^® as a surfactant, mixed solvent of C₂H₅OH and n-C₄H₉OH, and H₂O, followed by investigation of their structural and optical properties. Spherical particles with good morphology and comparatively high dispersibility were obtained from solutions with the molar ratio SPAN85^®:H₂O = 1:8-17.5, their sizes being 0.2-5 μm in diameter. Particles became small and aggregated when the molar ratio of H₂O was more than 17.5, but became large when it was less than 8. Spherical particles heat treated at 400°C or below were amorphous, and those heat treated at 600°C or above were crystallized. Tb³⁺-doped HfO₂ spherical particles showed green luminescence due to the ⁵D₄→⁷F_J (J = 6, 5, 4, 3) transitions of the Tb³⁺ ion, and relatively strong luminescence was observed both in dried gel particles and particles heat treated at 1000°C. These Tb³⁺-doped HfO₂ spherical particles can be expected for new optical materials.