Terahertz (THz) absorption spectra of the two crystalline forms of 5,5-diethylbarbituric acid (barbital) were measured with THz time-domain spectroscopy (THz-TDS). The spectra exhibited the dissimilarity between the two forms. Identification of the polymorphs and quantitative analysis of the two forms are possible by THz-TDS. Further, we performed ab-initio calculations of the vibrational modes considering the crystal structures of the two forms, and the results were in good agreement with the experimental data. We discuss the difference between the THz absorption spectra of the two forms.