[en] Highlights: • Spherical Ln³⁺-doped NaGdF₄ are obtained via a facile double solvent growth process. • Particle size decreases with the addition of another solvent. • Luminescent intensity is significantly enhanced with the decrease of particle size. • The energy transfer from Dy³⁺ to Eu³⁺ is directly observed. Different sizes of hexagonal spherical NaGdF₄:Eu³⁺ particles are synthesized via a facile hydrothermal method with the use of ethylene glycol (EG), propylene glycol (PG) or butylene glycol (BG) as another solvent. The particle size decreases with the addition of EG, PG or BG and the decreasing trend in BG/H₂O system is significantly more than that in the other two systems. Meanwhile, results show that luminescent properties of NaGdF₄:Eu³⁺ are enhanced along with the decrease of particle size. Besides, the energy transfer from Dy³⁺ to Eu³⁺ is directly observed in the PL spectra of NaGdF₄:Eu³⁺/Dy³⁺.