We report a facile oleic acid-assisted hydrothermal route to synthesize NaYF4:Yb/Er crystals doped by alkali ions (Li+ or K+ ions), and the effects of alkali ion doping on the crystal structure and upconversion (UC) fluorescence emission of the NaYF4:Yb,Er crystals are investigated in detail. With the increasing dosage of Li+ ions in the NaYF4:Yb/Er crystals, their morphologies are changed from rod to disk and finally to polyhedron. However, the morphologies of NaYF4:Yb/Er crystals doped with K+ ions almost keep the rod-like shape. In addition, the phase transition of LixNa1-xYF4 crystals initiates when x is equal to or larger than 0.5, but the phase of KyNa1-yYF4 remains unchanged even if y reaches 0.85. Furthermore, the UC emission intensity of NaYF4:Yb/Er crystals reduces with the doping of Li+ ions. On the contrary, the UC emission intensity of NaYF4:Yb/Er crystals increases as K+ ions are introduced. It is interesting that the green and red UC emissions of the K0.7Na0.3YF4:Yb,Er crystal are high 8 and 7 times, respectively. (C) 2013 Published by Elsevier B.V.