Well-dispersed YBO3:Ce3+, Tb3+ nanoparticles of 30-75 nm in size were prepared by a size-controllable solvothermal method in a 1,4-butanediol/water mixed solvent. The mean particle size could be increased from 32 to 73 nm by increasing the B/(Y + Ce + Tb) ratio in the starting materials from 1 to 3. Transmission electron microscopy, specific surface area analysis, and dynamic light scattering measurements confirmed that the nanoparticles were highly dispersed in solvent without any aggregation. Subsequently, the relationship between particle size and photoluminescence (PL) intensity was investigated. The nanoparticles showed green emission corresponding to 4f -> 4f transitions of Tb3+ under near-UV excitation. The PL intensity and quantum yield increased with increasing particle size, thus indicating that surface defects acted as the predominant PL quenchers within the studied size range.