CaF2 nanocrystals doped with Yb3+, Tb3+ and Li+ ions were investigated to study the correlation between their structural, electronic and up-conversion emission properties. The role of interstitial and substitutional Li+ ions in enhancing the up-conversion intensities were discussed. Evidence of Li+ at both interstitial and substitutional sites was provided by X-ray diffraction and X-ray photoelectron spectroscopy. The diffuse reflection spectra confirmed that the dopants were successfully incorporated. Up-conversion emission spectra under 980 nm excitation wavelength revealed the vital role of Li+ ions in enhancing both blue and green emission. The results reported during this investigation are expected to provide better understanding of the mechanism involved in enhancing the up-conversion emission in many Yb3+-Tb3+ co-doped fluoride materials due to Li+ ions doping.