The microwave spectrum of benzoic acid was observed in the frequency range 30∼40GHz by using a Stark modulation spectrometer and 6–10GHz by a molecular beam Fourier-transform microwave spectrometer (MB-FTMW). The rotational and centrifugal distortion constants of the normal and carboxylic deuterated species were obtained for the ground and C1–COOH torsional state. For the normal species in the ground state the rotational constants were determined by least-squares fitting using low J transitions observed with FTMW: A=3872.2741(5)MHz, B=1227.3128(2)MHz, C=932.5657(2)MHz. The obtained inertial defect (Δ=−0.3656(2)uÅ2) indicated that the equilibrium conformation was of the planar structure with large amplitude torsional motion. rs. Coordinates of the hydroxyl hydrogen were |a|=2.91(1)Å, |b|=1.14(1)Å, indicating the hydrogen was located close to the benzene ring.