Abstract: Herbs contain large amount of antioxidant such as secondary metabolites, which plays an important role in absorbing and neutralizing free radicals. A new Sesquiterpene, Isolongifolene is present in Murraya koenigii leaves. Murraya koenigii is a herbal species used in traditional medicine in Eastern Asia. Many diseases are associated with oxidative stress caused by free radicals. In this study, antioxidant activity of Isolongifolene was evaluated in a series of in vitro assay involving free radicals and EC50 values were determined. The EC50 value for in vitro antioxidant assays namely DPPH, ABTS, hydroxyl radical, nitric oxide, hydrogen peroxide, super oxide radical scavenging, FRAP,TRAP and Reducing Power, were 77.34, 40.9, 16.27, 238.3, 25.01, 16.79, 1.311, 6.701, 0.418 μg mL-1, respectively. Isolongifolene showed EC50 with minimum concentration and more effective in scavenging activities. Hence, based on the results we suggest that Isolongifolene compound can be recommended as a potent antioxidant to the patients suffering from various oxidative degenerative diseases like cancer.