Antioxidant activity of Caesalpinia sappan heartwood was studied both by in vitro and in vivo models. The ethyl acetate, methanol and water extracts exhibited strong antioxidant activity as evidenced by the low IC₅₀ values in both 1,1-diphenyl-2-picryl hydrazyl (DPPH) and nitric oxide methods. The values were found to be less or comparable to those of ascorbic acid and rutin, the standards used. Administration of the successive methanol and water extracts at 50 and 100 mg/kg body weight given for four days prior to carbon tetrachloride (CCl₄) treatment caused a significant increase in the level of superoxide dismutase (SOD) and catalase and a significant decrease in the level of thiobarbituric acid reactive substances (TBARS), when compared to CCl₄ treated control in both liver and kidney. These changes observed at 100 mg/kg body weight treatment were comparable to those observed for standard vitamin E at 50 mg/kg treatment. The results support significant antioxidant nature of Caesalpinia sappan heartwood extracts.