[en] We have newly developed ¹⁶⁶Er Mössbauer spectroscopy in Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS). As ¹⁶⁶Ho radioactive source, HoAl₂ was used. The compound was irradiated in the nuclear reactor (1 MW) for 5 min. The source has the intensity of 19 MBq and the half-life of 26.8 h. It is our privilege to conduct measurement immediately after the preparation of the radioactive source, since the spectroscopic equipment is situated just next to the reactor. The spectra of three typical Er oxides, Er₂O₃, ErFeO₃, and ErMnO₃ were measured at 40 K. On the basis of the calculated asymmetry parameter η and the evaluated sign of the quadrupole coupling constant e²qQ/2 from the structure parameters, we have successfully analyzed the spectra by using the quadrupole Hamiltonian assuming the transmission integral line shapes. The ¹⁶⁶Er Mössbauer parameters, isomer shift IS, e²qQ/2, and line width W were obtained for the three specimens.