The microstructures and upconversion luminescence of Er3+ single doped and Er3+/Yb3+ co-doped SiO2-Al2O3-CaO-NaF-CaF2 glass ceramics were investigated. Under the same heat treatment condition, the size of CaF2 nanocrystals precipitated in the co-doped sample was larger than that in the single doped one. The intense red and relatively weak green upconversion emissions were detected in both samples, while nearly nothing were detected in the corresponding as-made glasses under 980 nm excitations, implying that rare earth ions were incorporated into CaF2 nanocrystals, which was in accordance with the elemental analysis of the nanocrystals. The shortened distance between Er3+ ions is believed to enhance the energy transfers between them, resulting in the stronger red emission. A significant increase in the upconversion emission intensity in the co-doped sample due to high efficient energy transfers from Yb3+ to Er3+ ions was also observed. (c) 2006 Elsevier B.V. All rights reserved.