Nanocrystallization of ErF3 in oxyfluoride glass of composition 55SiO(2)center dot 20Al(2)O(3)center dot 15Na(2)O center dot 10ErF(3) (mol%) has been observed by its heat treatment above the glass transition temperature. The crystallization mechanism of this glass has been substantiated by the microstructural and compositional changes in the crystalline phase. Studies have been made through dilatometry, X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Maximum crystal size of 20 nm has been obtained, on variation of heat treatment time and temperature. Transmission electron microscopy (TEM) images of the as-annealed glass before heat treatment showed nonoccurrence of phase separation on quenching the melt. But after heat treatment for 10 h at 620 degrees C, ordered regions of dark contrast (0.5 nm) with respect to the matrix had appeared. These regions are believed to be associated with the process of onset of nucleation of fluorite phase. (C) 2013 Published by Elsevier Ltd and Techna Group S.r.l.