Yb~(3+) and Er~(3+) ions co-doped NaYF_4 nanocrystals were synthesized with different amounts of NaOH via oleic acid(OA)-assisted hydrothermal process. X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), Fourier transform infrared spectroscopy(FTIR), and photoluminescence spectra were used to characterize the products. The result indicated that the introduction of Na OH into the initial reaction solution effectively promoted the cubic(α-) to hexagonal(β-) phase transition of NaYF_4:Yb~(3+),Er~(3+) nanocrystals, while excessive amount of Na OH favored the formation of α-NaYF_4:Yb~(3+),Er~(3+) nanocrystals. Besides, with the increase of Na OH amount, the morphologies of β-NaYF_4:Yb~(3+),Er~(3+) nanocrystals varied from irregular nanobranch to uniform nanorod. Further investigation revealed that the addition of Na OH could facilitate the deprotonation of OA, leading to the formation of oleate(OA–), and meanwhile increased the concentration of OH– ions, inducing consequently the phase transition and morphology evolution of NaYF_4:Yb~(3+),Er~(3+) nanocrystals. Moreover, the upconversion luminescence properties of NaYF_4:Yb~(3+),Er~(3+) nanocrystals were systematically investigated. It was found that the upconversion emissions not only depended on the phase and morphology but also were influenced by the surface groups. Yb ~ (3+) and Er ~ (3+) ions co-doped NaYF_4 nanocrystals were synthesized with different amounts of NaOH via oleic acid (OA) -assisted hydrothermal process. X-ray diffraction ), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and photoluminescence spectra were used to characterize the products. The result indicated that the introduction of Na OH into the initial reaction solution actually promoted cubic (α-) to The results show that hexagonal (β) phase transition of NaYF_4: Yb ~ (3 +), Er ~ (3 +) nanocrystals, while excessive amount of Na OH favored the formation of α-NaYF_4: Yb ~ +) nanocrystals. Besides, with the increase of Na OH amount, the morphologies of β-NaYF_4: Yb ~ (3 +), Er ~ (3+) nanocrystals varied from irregular nanobranch to uniform nanorod. Further investigation revealed that the addition of Na OH could facilitate the deprotonation of OA, leading to the formation of oleate (OA-), and meanwhile increased the concentration on of OH- ions, inducing the phase transition and morphology evolution of NaYF_4: Yb ~ (3 +), Er ~ (3+) nanocrystals. Moreover, the upconversion luminescence properties of NaYF_4: Yb ~ (3+) nanocrystals were systematically investigated. It was found that the upconversion emissions not only depended on the phase and morphology but also were influenced by the surface groups.