Structure of pyrazol-5-one , 3-methyl pyrazol-5-one and 1-phenyl-3-methyl pyrazol-5-one and their IR spectra data have been determined by performing DFT molecular orbital calculations at B3LYP/6-31G(d) theory level . The geometry parameters, total energies, HOMO and LUMO orbitals energies, dipole moments and changes on the atoms as well as vibrational frequencies were calculated for the three molecules and the results were compared with the experimental measurements and discussed . The NMR chemical shifts were determined using TNDO/6-31G(d) method . The calculations showed that pyrazol-5-one is aplanar molecule and N-H vibrational frequency is lower than that in succimide . The methyl and phenyl substituents decrease N-H bond vibrational frequency whereas C=O frequency increase by phenyl substituent