The molecular structure of trans-azobenzene (Ph-N=N-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 shrink-age 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(phi (1),phi (2)) = Sigma (i=1,2){V-2(1 - cos 2 phi (i))/2 + V-4(1 - cos 4 phi (i))/2}, where phi (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(phi (1),phi (2)). Because P(phi (1),phi (2)) = N exp(-V(phi (1),phi (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)/Angstrom, angle (alpha)/deg) with the estimated limits of error (3 sigma) are as follows: V-2 = 1.7(6); V-4 = 0.6(13); r(N=N) = 1.260(8); r(N-C) = 1.427(8); = 1.399(l); = 1.102(7); angle NNC = 113.6(8); (angle NCCcis - angle NCCtrans)/2 = 5.0(9), where < > means an average value and C-cis and C-trans denote the carbon atoms cis and trans to the N=N 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)=N-Ph) and trans-stilbene (Ph-CH=CH-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.