The bonding character, electron delocalization, and aromaticity of C 18 precursors, C 18 -(CO) n (n = 6, 4, and 2), are theoretically explored in depth by using DFT calculations and a variety of wavefunction analyses methods. The optimized geometry and various bond orders indicate that the cyclocarbon skeleton of the precursors shows an alternating structure of long and short C-C bonds, which is akin to C 18 . The results of MO analysis and IRI analysis show that these molecules have two sets of π-conjugated electron systems, respectively described by π out and π in MOs, which provide different contributions to molecular aromaticity. The orbital interaction analysis reveals the essential reason why the introduction of -CO groups destroys the π in conjugated system. Based on the analysis of LOL- and ELF- functions, the characteristics of electron delocalization in π out and π in MOs were confirmed. The ACID, GIMIC, and ICSS ZZ methods graphically revealed the prominent induced ring current or magnetic shielding effect of  electrons in precursors induced by external magnetic field. All the analysis methods based on the wavefunction of quantum chemistry calculations or response to external magnetic field came to exactly the same conclusion, that is, the three precursor molecules all have π out electron global delocalization and π in electron local delocalization, so they can be definitely classified as aromatic species, and the aromatic strength increases with the decrease of number of -CO groups.