Fruit pulp powder of Cassia fistula was investigated for its antioxidant activity both in vitro and in vivo. Preliminary phytochemical analysis showed high phenolic and flavone content (22 mg/kg and 4 mg/kg respectively) in pulp. A concentration dependent, increase in FRAP value obtained; suggest high antioxidant property of the extract. In vivo study in young adult mice fed fruit pulp powder extract (100 mg/ kg/BW single dose daily for 30 days) one hour prior to Combination of stresses (immobilization followed by swimming type) of 2 hrs duration daily up to 30 days, showed significant increase in superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and reduced glutathione (GSH) levels in brain, gastronomies muscle, heart, kidney, lung and stomach of these mice as compared to mice given only stress. Levels of malondialdehyde (MDA) were significantly lowered after the drug treatment in all the tissues in stress group mice as compared to control. High antioxidant activity of Cassia fistula may be contributed to its high phenolic and flavonoid content.