Ab initio Mo calculations were applied to hydrazine to obtain a reliable molecular force field and to clear up some complexities in the observed spectrum and contradictions in the assignments reported by previous investigators. Various levels of ab initio MO methods, namely the Hartree-Fock (HF) SCF methods with STO-3G, 4-31G, and 4-31G ∗ (in which polarization functions of the nitrogen atoms were added) basis sets, and the configuration interaction (CI) method with a 6-31G ∗ basis set were compared with one another with respect to the calculated molecular parameters. It has been concluded that the HFSCF-4-31G ∗ is, in general, significant and useful, and the improvement caused by elevating the level of calculation up to CI-6-31G ∗ is not very great. For instance, most of the off-diagonal force constant values calculated with HFSCF-4-31G ∗ are already found to be reliable. On the other hand, however, with the 6-31G ∗ CI method, and with anharmonic corrections, nearly perfect reproduction of the observed fundamental frequencies of N 2H 4 has been achieved. Guided by this calculation, a few pieces of new interpretation of the N 2D 4 spectrum have been presented.