High quality NaYF : Yb , Er /SiO core-shell structures with different sizes were prepared by coating silica shell onto 100 nm size hydrophilic and 20 nm size hydrophobic up-conversion nano particles through hydrolysis of TEOS using ethanol and cyclohexane as main solvent respectively. Effects of reaction time and precursor concentration on the synthesis of core-shell structure were studied. The results show that the core shell structure could be easily dispersed in polar solvent with uniform silica coating on hydrophobic and hydrophilic, big and small NaYF : Yb , Er nano particles. Up-conversion spectrum characterizations of the nano structure were conducted under room temperature. There were no changes of the emission intensity and emission center for particles before or after silica coating. Green light around 550 nm and red light around 670 nm could be observed with 980 nm laser excitation.