Sub-25 nm β-NaLuF₄ nanocrystals (NCs) with high homogeneity, monodispersity, and good crystallinity were synthesized through thermal decomposition at different temperatures (285, 295, 305, and 315 °C), using an automatic nanomaterial synthesizer. The phase transition of NaLuF₄ NCs from α-phase to β-phase suggests that the entire growth process consists of four stages: a cubic phase, the coexistence of cubic and hexagonal phases, a homogeneous hexagonal phase, and an inhomogeneous hexagonal phase, caused by Ostwald ripening. Based on this observation, we obtained the growth phase diagram of NaLuF₄:Yb³⁺/Tm³⁺/Gd³⁺ NCs, relating to the reaction time, temperature, crystal size, and crystal phase. From spectral analysis, the products prepared in the third stage were shown to exhibit the characteristics of having small size (sub-25 nm), uniformity, and high upconversion luminescence (UCL) intensity. On the other hand, the luminescence properties of NaLuF₄:Yb³⁺,Tm³⁺ were studied via incorporating different concentrations of Gd³⁺ ions. These results indicated that 10% Gd³⁺ was the best doping concentration for obtaining NaLuF₄ NCs with relatively small size and high brightness. Further increasing the Gd³⁺ concentration induced a drastic decrease in UCL intensity.