Five new sesquiterpenes were isolated from Acorus calamus L., in addition to calameone (or calamendiol), the structure of which was revised, and their structures were established. Furthermore, chemical co-relation among these sesquiterpenes was carried out. In particular, the thermal isomerization of shyobunone (I), an elemene-type sesquiterpene, to preisocalamendiol (VI), a germacrone-type compound