The present invention relates to a catalytic process for preparation of isolongifolene using nanocrystalline solid super acid. This process is an eco-friendly, single step, solvent free catalytic process for the preparation of a tricyclic sesqui-terpene hydrocarbon, isolongifolene. More particularly, the present invention provides a process for the catalytic isomerisation of longifolene to iso-longifolene using nano-crystalline sulfated zirconia as a solid super acid catalyst.