The morphology and spatial distribution state of CaF₂ nanocrystals in oxyfluoride glass-ceramics based on the ErF₃–CaF₂–Al₂O₃–SiO₂ system with different ErF₃ contents of 1–3 mol % and the addition of AlN were examined from transmission electron microscope (TEM) observations. The content of 3 mol % ErF₃ induced the growth of CaF₂ nanocrystals with star-like morphologies and their assemblies with the size of ~70 nm. The addition of a small amount (2 mol %) of AlN has a remarkable effect on the spatial uniform distribution (i.e., disappearance of assembly) of CaF₂ nanocrystals. The present study proposes a new method for the design of composition in rare-earth doped oxyfluoride glass-ceramics.