Efficient upconversion (UC) microcrystals are important for the development of novel photonic markers, solid-state lasers, displays, etc. Herein, we report on greatly enhanced UC emission in beta-NaYF4:20Yb,2Er microcrystals via a simple ion-exchange reaction. The ion exchange accompanying with an ionic redistribution process is demonstrated. By simply controlling the amount of NaF (0-8.0 mmol), reaction temperature (140-220 A degrees C), and duration (0-144 h), enormous enhancement of the UC emission up to similar to 4200 times is obtained compared with the pre-synthesized beta-NaYF4:20Yb,2Er microcrystals. The decrease of body defects and change of local structure around lanthanide may account for the UC emission enhancement. This method may apply to other UC materials to increase the UC emission efficiency.