Abstract: A new sesquiterpene compound named isorupestonic acid was isolated from Artemisia rupestris and its absolute stereostructure was elucidated as 3B by spectral and X- ray crystallographic methods. Interestingly, the isorupestonic acid is based upon an unusual 6, 7 membered ring skeleton differing from the 5, 7 membered ring rupestonic acid although both have the same absolute configuration at C-1. The biogenesis of isorupestonic acid most likely involves breaking the C₄-C₅ bond of rupestonic skeleton and forming the C₅- C₁₄ bond.The absolute configuration of rupestonic acid was also determined as 4C by X-ray analysis and CD data.