We observed experimentally that the formation of beta-PbF(2) nanocrystals (NC) in oxyfluoride glass conspicuously altered the local structures of rare-earth (RE) ions, which then enabled their intra4f-configurational transitions to be luminuous even at mid-infrared (MIR) wavelengths. Radiative emission centered at similar to 2.87 mu m originating from Dy3+:6H(13/2)-> 6H(15/2) transition was remarkably enhanced after the NC were formed by thermal treatment in an ambient atmosphere. However, emission intensities due to Er3+:4I(11/2)-> 4I(13/2) and Ho3+:5I(6)-> 5I(7) transitions turned out to be less dependent on the NC formation, which indicated that the enhancement of MIR emission depended on RE.