The Infrared (IR) and Raman spectra of 5-nitro-2,4-dihydro-1,2,4-triazole-3-one (NTO) and its compounds labeled with the stable isotopes N-15, C-13, and deuterium (D) (such as NTO-(NO2)-N-15, NTO-C-13(3) or NTO-D) were measured in the region of 100-3500 cm(-1). Vibrational motions were experimentally assigned by using isotopic vibrational shifts. In addition, theoretical calculations were performed to assign those vibrational bands with good agreement of experimental and theoretical data. Furthermore, the experimental and calculated results indicate that the C-O bond length of NTO in the crystal state is longer than that of an isolated molecule in the gas phase because of intermolecular hydrogen bonding.