The Huckel molecular orbital spectra (HMO) of Huckel- and Mobius-type cyclacenes having 3-15 benzenoid rings were investigated and it was found that certain eigenvalues are topologically invariant, whereas others exhibit topological periodicity. Fbr example, Huckel-type cyclacenes having 2r (r: 2,3,4,...) benzenoid rings which possess nonbonding molecular orbitals (NBMO) and are thus expected to be open shell structures.