Sm3+ and Er3+ co-doped NaYF4 microcrystals were synthesized through a facile hydrothermal method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), upconversion (UC) emission spectra, and dynamic analysis. In the co-doped samples, the bridging effect of Er3+ ions played an important role in populating excited Sm3+. The energy transfer (ET) from Er3+ to Sm3+ was confirmed conclusively under 383 nm excitation. Experiments on concentration variation and dynamic analysis revealed the ETs from Er3+ to Sm3+ in detail. The possible routes for populating excited Sm3+ ions were proposed based on spectral and dynamic analysis. (C) 2012 Elsevier B.V. All rights reserved.