Abstract: Versicolactone C has been isolated from the root of Aristolochia versicolar S.M. Hwang. It is a new type of sesquiterpene lactone with hydroazulene skeletone, C_(15)H_(22)O_4, mp 181-182℃. In this paper its MS fragmentation pattern has been proposed on the basis ot the data from high resolution technique and metastable ion measurements.