Fruit, stem bark and leaves of Cassia fistula Linn. Retain a biologically active compound such as flavonoids, glycosides, tannin, anthraquinone, saponin, steroids and alkaloids and many other compounds. The stem bark, fruit and leaves extract shows various activities like antipyretic, ant inflammatory, antidiabetic, analgesic, laxative, hypolipidemic, hepatoprotective, anti-ulcer, antitumor, antimicrobial, antifungal and antioxidant. Traditionally used in many problems in form of lotion, syrup, quath, powder etc. The main aim and objective of this study is to find out the phytochemical constituents responsible for antioxidant activity of Cassia fistula Linn. Barks extract and evaluate the total antioxidant activity by reducing power assay. Activity of Cassia fistula bark methanol extract (CFBE) on hematological parameter of Swiss albino mice induced by radiations is also assess.