Terahertz (THz) spectra of bentazon are determined within the range of 0.3 2.4 THz at room temperature. Density functional methods are used to compute the THz spectra using three different programs: Gaussian03 for isolated-molecule form, DMol3 and CRYSTAL09 for solid-state forms. Among the three, the computed THz spectrum of CRYSTAL09 shows better bond length and angle agreements with X-ray experimental results, and corresponds with observed THz experiment spectral characteristics. The isolated-molecule vibrational mode values are less by half than those derived from solid-state calculations. The last five peak positions of the two solid-state computations coincide with each other. Moreover, all the experimental THz absorption peaks are assigned by utilizing CRYSTAL09.