A series of annular structures of C-n (3 <= n <= 31) are optimized at DFT/B3LYP/6-31G* theoretical level. The calculated results show that odd structures and even structures have different bonding characters and properties. NBO analyses indict that C-6, C-10, C-14, C-18, C-22, C-26 and C-30 molecules have two big delocalized pi-bonding and C-8, C-12, C-16, C-20, C-24 and C-28 molecules have only one big delocalized pi-bonding. C-4 has aromaticity for it has different conformation which makes it have special bonding character. In addition, linear C-n structures are calculated to further investigate the nature of annular C-n, structures. (c) 2006 Elsevier B.V. All rights reserved.