Abstract We presented a series of quantum-chemical calculations of [18]annulene (C18H18) and its analogues which are helically distorted open-chain molecules and which we call “quasi-Möbius-type” molecules in this paper. We discussed the proton magnetic shielding constants, the chemical shifts, and the nucleus-independent chemical shift (NICS) of these molecules. The calculated nuclear magnetic shielding constants were decomposed into the diamagnetic and paramagnetic terms using a conventional manner. As we expected, a shielding–deshielding reversal occurs in C18H18 and quasi-Möbius-type C18H20. Moreover, we showed a predictive demonstration that the magnetic shielding constants can be controlled by inserting a small molecule such as CH3− or H2C=CH2 into the terminal π-orbitals of quasi-Möbius-type C18H20 or C16H18. Aromaticity and antiaromaticity are discussed with magnetic features of molecules.