Abstract: The aim of this study was to evaluate the antioxidant potential of aqueous leaf extract of Anacardium occidentale (AOE) against hydrogen peroxide (H2O2)-induced oxidation on human umbilical vein endothelial cells (HUVECs). HUVECs were exposed with various concentrations of H2O2 (0-700 μM) and it was observed that 250 μM of H2O2 reduced cell viability by 50% (inhibitory concentration, IC50) as denoted by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay. Using the above concentration as the positive control, the cells were pretreated with AOE at various concentrations (50-700 μg mL-1) for 30 min, followed by 24 h incubation with H2O2 (250 μM). The AOE (100-300 μg mL-1) was found to protect cellular damage and prevent microsomal lipid peroxidation in H2O2-induced HUVEC indicated with low MDA levels. Furthermore, AOE at similar concentrations may directly act on H2O2 as indicated with reduction of Superoxide Dismutase (SOD), Catalase (CAT) and Glutathione Peroxidase (GPx) activities. Overall, these data suggested that AOE possessed antioxidative properties against H2O2-induced oxidation on HUVECs and improved the endogenous antioxidant enzymes activity in vitro.