We investigated the Antioxidant activity of isolated compound 2,3,6 trimethyloct-6-enal from methanol extract of Indian medicinal plant Pajanelia longifolia (Willd.) K. Schuman.The Antioxidant activity of 2,3,6 trimethyloct-6-enal was evaluated against CCl4 [0.5 ml/kg body weight intra-peritoneal (b.w. i.p.)] induced hepatic damage in Swiss albino mice. 2,3,6 trimethyloct-6-enal at dose level of 100, 200 and 300 mg/kg body weight per orally (b.w.p.o.) respectively were administered to mice before 30 min. of CCl4 administration. Silymarin at a dose of 50 mg/kg b.w.p.o. was taken as a standard drug. The antioxidant activity of 2,3,6 trimethyloct-6-enal was measured on antioxidant enzymatic and non-enzymatic levels (such as reduced glutathione (GSH), lipid peroxidation (LPO), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) ). Further the effects of 2,3,6 trimethyloct-6-enal on serum biochemical parameters, were also estimated such as the serum glutamic pyruvic transaminase (SGPT),serum glutamic oxaloacetic transaminase (SGOT), serum alkaline phosphatase (SALP) and bilirubin (total).The HPLC result was clearly indicated that the compound was pure.Considering the Spectral data (IR spectrum, 1HNMR spectrum and13C NMR spectrum) of 2,3,6 trimethyloct-6-enal the predicteble structure was also elucidated.