Conversion pathways of prism-C-16 with a D-8h structure to a monocyclic C-8h structure on the potential energy surfaces (PES) were elucidated by quantum chemical calculations. Eight four-membered rings on the side faces of prism-C-16 were broken successively to yield finally a monocyclic C-8h ring, which was found to be the global minimum of C-16. Quantum chemical exploration of PES gave more than two thousands of isomeric structures and transition structures. Intrinsic reaction coordinates along reaction pathways were studied to obtain reaction mechanisms from the linear C-16 structure to cyclic forms.