[en] Low-temperature molten-salt method and the NH₄NO₃ flux were developed to synthesize quasi-spherical cubic BaGdF₅:Ce³⁺/Er³⁺/Yb³⁺ nanocrystals with the average diameter of 30 nm. The optimum doping concentrations of Er³⁺ and Yb³⁺ are determined to 2 and 20 mol%, respectively. The introduction of Ce³⁺ in BaGdF₅: Er³⁺/Yb³⁺ nanocrystals deduced a huge variation of the profile of the UC emission under 976 nm laser irradiation. The strong ultraviolet UC emission of 330–380 nm is due to the overlap of ⁴G_7/2,²K_15/2,⁴G_9/2 → ⁴I_15/2 transitions of Er³⁺, and strong blue one locating at 440–480 nm is attributed to the overlap of ⁴F_3/2,⁴F_5/2 → ⁴I_15/2 and ⁴G_7/2,²K_15/2,⁴G_9/2 → ⁴I_13/2 transitions of Er³⁺, respectively. The UC emission mechanism and energy transfer process of BaGdF₅:Ce³⁺/Er³⁺/Yb³⁺ nanocrystals were provided in detail and demonstrated by the fluorescence decay curves and pumping power dependent UC emission spectra. This work not only demonstrates a rational approach to tailor UC emission (red, blue and ultraviolet) by tuning doping ions and concentration, but also for better understanding of their essential UC luminescence properties. - Highlights: • The high-quality cubic BaGdF₅:Ce³⁺/Er³⁺/Yb³⁺ nanocrystals were synthesized by low-temperature molten-salt method in NH₄NO₃ flux. • The UC emission mechanism was demonstrated in detail by the fluorescence decay curves. • Tailoring UC emission of Er³⁺ was obtained by tuning Ce³⁺ concentration.