Crystal structures and dynamic behavior in solution of some aromatic N-methylanilides were analyzed. Every N-methylamide examined was in cis form in the crystal, and exhibitedcis -preference in solution. Among them, meta-substituted amides, such asN,N ,N -trimethyl-N,N ,N -triphenyl-1,3,5-benzenetricarboxamide (5), exist in the all-syn conformation, in which all the N-phenyl groups are on the same side of the plane of the central benzene ring in the crystal. 1 H-NMR studies suggested that 5 existed mainly in all-syn conformation at low temperature as found in the crystal, while the anti conformation, in which one N-phenyl group is on the opposite side of the central benzene ring, becomes predominant at higher temperature, and a rapid equilibrium exists between the syn and the anti conformations. This is a result of the positive enthalpy (ΔH° = + 2.1 kcal/mol) and the identical-signed large entropy (ΔS° = + 8.4 cal/mol K) for the conversion from syn to anti conformation. From NMR experiments, the enthalpy gained by assembly of the three phenyl groups in solution was deduced to be 2.8 kcal/mol.