The Er3+ concentration dependencies of upconversion luminescence in oxy-fluoride glass and glass-ceramics containing PbF2 nanocrystals were investigated. Strong red emission from the (F9/2I15/2)-F-4-I-4 transition was observed with the addition of similar to 0.8mol% Er3+ ions, whereas similar to 10mol% of Er3+ is required to achieve such emission in several other crystalline hosts. Intensities of red emission further increased with the formation of nanocrystals through heat treatment. The Er3+ ions enriched in glass and segregated preferentially inside the PbF2 nanocrystals that decreased the distance among Er3+ ions and thereby facilitated energy transfer.