Conformational flexibility of exhaustively methylated azacalix[4]arene in solution has been investigated. Relaxation time measurements revealed that its 1,3-alternate conformation was inflexible even in solution. Intramolecular small annulus plays a central role in suppressing the conformational inversion by the small but yet sufficiently bulky O-methyl groups. Without regard to the small anulus, the azacalix[4]arene has a room to selectively accommodate potassium ion in the cavity.