Making orthodontic adhesives visible is valuable for safe and complete removal of these adhesives after completion of orthodontic treatments using a multibracket appliance. In order to make orthodontic adhesives visible by irradiation with purple or near ultraviolet light, trivalent europium ions, which emit clear red fluorescence, were doped into the lattices of zinc oxides by a homogeneous precipitation method and subsequent calcination at 400℃ or 850℃. The primary crystals of the synthesized products, overall, were on the nanoscale. The fluorescence intensity of the ZnO:Eu^<3+> nanoparticle-containing polymer adhesives was somewhat weak. The spectra, independent of the calcination temperatures, exhibited characteristic peaks corresponding to the intrinsic 4f-4f transitions of Eu^<3+>. However, it is evident from the X-ray diffraction (XRD) patterns that the 850℃-calcination tended to crystallize more EU_2O_3, along with increased incorporation of Eu^<3+> ions into ZnO lattices, in comparison to the case with the 400℃-calcination. In addition, although the yielding load of the adhesives containing nanoparticles calcined at 850℃ decreased significantly, the decrease in the actual measured values was quite limited in both the cases. From a comprehensive standpoint, although the ZnO:Eu^<3+> nanoparticles calcined at 850℃ exhibited acceptable fluorescence intensity, the calcination at 400℃ is preferred for application to orthodontic adhesives.