Tetrathiocin (tetrathiacycloocta-3,7-diene) 1, substituted by sulfur atoms at the sp^3 carbons of cycloocta-1,5-diene, and octathiacyclohexadeca-3,7,11,15-tetraene 2 were synthesized successfully by oxidation of cis-disodium ethene-1,2-dithiolate. The X-ray structure of 1 showed the twist form (nearly D_2 symmetry), and the crystal structure of 2 was found to have cage geometry (nearly D_<2d> symmetry) possessing a cavity. The cage structure indicates that the compound 2 is expected to become a new inclusion compound with two types of coordination site (pi-electrons of the olefins and lone pairs on the sulfur atoms). A comparison of 1 was also studied based on ab initio MO calculations. The twist conformer of 1 was calculated to have the lowest potential energy among all the conformers, and the most stable geometry showed good agreement with the X-ray structure of 1. Furthermore, stable geometries were found to be different between 1 and corresponding carbon analogue (cycloocta-1,5-diene). Tetrathiocin 1 was thermally stable and no decomposition was observed even in refluxing p-xylene after 3 h. However, compound 1 changed in polar solvent, such as acetonitrile/chloroform solution, even at room temperature to give the ring expansion product 2 in good yield. Compound 1 was also found to be unstable toward irradiation, even room light. Irradiation of 1 in a benzene solution, a rubber-like yellow polymer was formed together with bicyclic compound.