The antioxidant potency of different fractions of Moringa oleifera leaves were investigated by employing various established in vitro systems, such as beta-Carotene bleaching, reducing power, DPPH/superoxide/hydroxyl radical scavenging, ferrous ion chelation and lipid peroxidation. On the basis of in vitro antioxidant properties polyphenolic fraction of M. oleifera leaves (MOEF) was chosen as the potent fraction and used for the DNA nicking and in vivo antioxidant properties. MOEF shows concentration dependent protection of oxidative DNA damage induced by HO() and also found to inhibit the toxicity produced by CCl(4) administration as seen from the decreased lipid peroxides (LPO) and increased glutathione (GSH) levels. Among the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) levels were restored to almost normal levels compared to CCl(4) intoxicated rats. The HPLC analysis indicated the presence of phenolic acids (gallic, chlorogenic, ellagic and ferulic acid) and flavonoids (kaempferol, quercetin and rutin). Thus, it may be concluded that the MOEF possess high phenolic content and potent antioxidant properties, which may be mediated through direct trapping of the free radicals and also through metal chelation.