In this study, spherical YAG:Ce³⁺ nanophosphors (NPs) with a particle size of 200 nm were successfully synthesized by a solvothermal method using triethylene-tetramine (TETA) as a reaction solvent. Besides as the reaction medium, TETA also acted as the precipitant for the formation of hydroxide precursors at the initial stage of the solvothermal reaction. The shape of the particle prepared with 4 mL of TETA as the precipitant is a more regular sphere with a larger size (about 500 nm). The phase compositions, microstructures and photoluminescent properties of the as-prepared YAG:Ce³⁺ NPs were investigated via X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a fluorescence spectrophotometer. To improve the PL properties of the as-prepared YAG:Ce³⁺ NPs, the holding time, Ce³⁺ doping concentration and filling factor of the autoclave have been systematically optimized. The optimal cerium doping concentration is 3 mol%.