The molecular structure of trans-azobenzene (Ph-NdN-Ph) has been determined by gas electron diffraction. Diffraction patterns were taken at 407 K and data analysis was made using the structural constraints obtained from MP2/6-31+G* calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the harmonic force constants given by a normal coordinate analysis. Vibrational mean amplitudes were refined as groups. The torsion of each phenyl ring was treated as a large amplitude vibration. The potential function for torsion was assumed to be V(φ 1 ,φ 2 ), where φ i denotes the torsional angle around each N-C bond. Quantum mechanical calculations were performed by taking account of two torsional motions to derive a probability distribution function, P(φ 1 ,φ 2 ). Because P(φ 1 ,φ 2 ) ) N exp(-V(φ 1 ,φ 2 )/kT) was found to be a good approximation at 407 K where N is a constant, it was adopted in the data analysis. The determined potential constants (V 2 and V 4 /kcal mol -1 ) and principal structure parameters (r g /Å, ∠ R /deg) with the estimated limits of error (3σ) are as follows: V 2 ) 1.7(6); V 4 ) 0.6(13); r(NdN) ) 1.260(8); r(N-C) ) 1.427(8); ) 1.399(1); ) 1.102(7); ∠NNC ) 113.6(8); (∠NCC cis -∠NCC trans )/2 ) 5.0(9), where < > means an average value and C cis and C trans denote the carbon atoms cis and trans to the NdN bond, respectively. Thus, the stable form was found to be planar with C 2h symmetry. The observed structure was compared with those of trans-azoxybenzene (Ph-N(-O)dN-Ph) and transstilbene (Ph-CHdCH-Ph). The stability of the liquid crystals with these types of molecular cores was discussed on the basis of the gas-phase structures of the model compounds of cores. Nearly the same results were obtained in the data analysis using the constraints from RHF/6-31G** ab initio calculations.