The infrared spectra of uranium oxides (UO_x) of various O/U ratios (x = 2–2:667 in UO_x) were measured by using ATR-FTIR in order to investigate the applicability of ATR technology to the characterization of uranium oxides or nuclear fuel. The samples of uranium oxides (UO_x) were prepared by heating UO₂ powder in air atmosphere followed by weighing to determine the gains by oxygen. X-ray diffraction (XRD) patterns for the same samples of infrared spectra were measured to get information on their phases on uranium oxides (UO_x) with increasing x. The measured characteristic bands from UO_x samples could be assigned well by referring to the spectra reported in the literature and comparing with the diffraction patterns measured. The relationship between absorbance and O/U ratio (x) was determined. We discussed about using it for the O/U ratio estimation of uranium oxides of unknown composition. It was confirmed that ATR technology for infrared spectroscopy can be useful for the studies of radioactive materials that are difficult to handle using conventional methods.