The objectives of this study was to examine the antioxidant activities of aqueous extracts of bark of Psidium guajava bellowing to family Myrtaceae, which is originated in Mexico and extends all over the South America, Europe, Africa and Asia. The Psidium guajava is used traditionally  for the treatment of various ailments like  antioxidant, hepatoprotective, anti-allergy, antimicrobial, antigenotoxic, antiplasmodial, cytotoxic, antispasmodic, cardioactive, anticough, antidiabetic, anti-inflammatory and antinociceptive activities. For the antioxidant activity of aqueous extract of bark of Psidium guajava Linn, In-vitro and In-vivo methods are used. In-vitro assessment of the ability of the extract to scavenge the Hydrogen peroxide (H2O2) was determined with reference to the synthetic antioxidant vitamin C and ascorbic acid was used as standard antioxidants. The scavenging activity of plant extract on hydrogen peroxide was found similar as scavenging activity of vitamin C. The Psidium guajava showed the strong In-vitro antioxidant activity and exhibited over 86.01% inhibition at 12 µg/ml concentration. For In-vivo antioxidant activity catalase (CAT) enzyme level was used. CAT levels were measured in fresh liver tissue using ELISA. CAT in hepatic tissue compared to control group. CAT enzymes were significantly increased when treated with aqueous extract of bark of Psidium guajava. In human beings, the highest levels of CAT are found in the liver, kidney and erythrocytes, where it is believed to account for the majority of H2O2 decomposition. These results show that the Psidium guajava could be considered as a natural antioxidant source.