In this letter, we employ terahertz time-domain spectroscopy (THz-TDS) technology to measure the THz absorption spectrum and refractive index of benzoic acid at room temperature. Linearity combination of atomic orbital (LCAO) within the density functional theory (DFT) is also used to compute the vibrational absorption spectra of benzoic acid in monomer and dimer. In addition, based on the results obtained from density functional calculation, the origins of observed intra-molecular and inter-molecular vibrational modes under specific peaks are interpreted.