The geometrical and electronic structures of large cyclic polyenes, annulenes, with 18, 30, 42, 54, and 66 CH units, which belong to a subgroup of aromatic (4n + 2)pi electron systems and can exhibit a delocalized D-6h geometry, are studied using the MNDOC (C for correlation) method coupled with a conventional perturbation treatment. The optimized D-6h structures have nearly equal C-C bond lengths of approximately 1.4 Angstrom, in contrast to the D-3h ones which exhibit strong bond-length alternation. It is well-known that the Hartree-Fock-based self-consistent-field (SCF) methods generally prefer bond-alternation structures to bond-equalization ones. However, second-order energy (always minus) is larger in the D-6h structures, due to the small HOMO-LUMO gap in the D-6h structures with nearly equal C-C bond lengths. It is clarified that these two effects strongly compete in the structure of [18]- and [30]annulenes; in [18]annulene the D-6h structure is 6.4 kcal/mol more favorable, but in [30]annulene the D-6h structure is less stable by 4.6 kcal/mol. Moreover, in [42]-, [54]-, and [66]annulenes, the D-3h structures are clearly more stable than the corresponding D-6h ones.