Ab initio Hartree-Fock and density functional theory calculations were carried out to investigate the structure and vibrational spectrum of 1,3-dithiole-2-thione. All the calculations predicted a planar structure with C2v symmetry. Harmonic force field and vibrational mode calculations provided convincing theoretical evidence for reassignments of some fundamental vibrational modes. The reassignments are in line with the observed polarization data of Dyer et al. This study shows that the density functional theory is a powerful tool for understanding the vibrational spectra of organic molecules.