Antioxidant activities of the 95% ethanol extract from Caesalpinia sappan heartwood (ECS), pro-tosappanin A, protosappanin B, and brazilein were studied in vitro. The inhibition of the formation of malondialdehyde (MDA) and the scavenging of superoxide anions, hydrogen peroxide, and hydroxyl radicals were assayed. The experimental results show that all four substances had antioxidant activity in vitro but their capabilities differed for the different indicators. ECS, protosappanin A, and protosappanin B show more inhi-bition of MDA and scavenging of hydrogen peroxide, while brazilein shows more scavenging of hydroxyl radicals. All the samples show little scavenging of superoxide anions. Antioxidant activities of the 95% ethanol extract from Caesalpinia sappan heartwood (ECS), pro-tosappanin A, protosappanin B, and brazilein were studied in vitro. The inhibition of the formation of malondialdehyde (MDA) and the scavenging of superoxide anions, , and the hydroxyl radicals were assayed. The experimental results show that all four substances had antioxidant activity in vitro but their capability differed for the different indicators. ECS, protosappanin A, and protosappanin B show more inhi-bition of MDA and scavenging of hydrogen peroxide, While brazilein shows more scavenging of hydroxyl radicals. All the samples show little scavenging of superoxide anions.