We investigated the influence of the composition of a 1,4-butanediol–water mixed solvent on the structural and particulate properties in the synthesis of well-dispersed YBO₃:Ce³⁺,Tb³⁺ nanocrystals, without assembly, using a solvothermal method. YBO₃:Ce³⁺,Tb³⁺ nanocrystals were synthesized from trimethyl borate and acetates of yttrium, cerium(III), and terbium(III) via a solvothermal reaction at 250 °C for 6 h in the mixed solvent. The presence of water promoted YBO₃ crystallization. The size of the primary nanocrystals increased from ∼5 to ∼40 nm as the water content of the mixed solvent increased from 2.5 to 100 vol%. The primary nanocrystals formed disk-like secondary assembled particles for water contents below 75 vol%, but formed polyhedral secondary particles in pure water. In contrast, well-dispersed primary nanocrystals with a mean hydrodynamic size of ∼35 nm were obtained when the water content was 75 vol%. These results indicate that not only the size of the primary nanocrystals, but also their assembly can be controlled by changing the composition of the mixed solvent. The obtained well-dispersed nanocrystals were transparent to the naked eye and showed green photoluminescence corresponding to 4f → 4f transitions of Tb³⁺ by energy transfer from Ce³⁺ to Tb³⁺ under near-UV irradiation.