Oxidative stress and inflammatory mediators were measured in the plasma and livers of C57BL/6 mice fed a high-cholesterol diet for 14 weeks and in cultured human umbilical vein endothelial cells (HUVECs). Some of the mice fed with the atherogenic diet received drinking water supplemented with 0.01 g of a 70% ethanol extract of Caesalpinia sappan L. (CSLE) per 20 g of body weight. Numerous parameters were determined: concentrations of total, high-, and low-density cholesterol; atherogenic index; plasma trolox equivalent antioxidant capacity (TEAC); levels of hepatic thiobarbituric acid reactive substances (TBARS) and protein carbonyls; and the activities of hepatic antioxidant enzymes, including Cu·Zn-SOD, Mn-SOD, glutathione peroxidase, glutathione reductase, and catalase. HUVECs were stimulated with tumor necrosis factor α (TNFα) and the expression of intracellular reactive oxygen species (ROS), lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), adhesion molecules, inhibitory κBα (IκBα), and nuclear factor κB (NFκB) were measured. Compared to mice fed a hypercholesterolemic diet alone, mice fed a hypercholesterolemic diet supplemented with CSLE exhibited decreased total plasma cholesterol and increased high-density lipoprotein cholesterol, and thus a lower atherogenic index. Furthermore, plasma TEAC and levels of hepatic TBARS and protein carbonyls were significantly decreased in CSLE-supplemented mice (P < 0.05), whereas all hepatic antioxidative indicators were significantly elevated (P < 0.05). In HUVECs stimulated with TNFα, CSLE significantly decreased the expression of intracellular ROS, LOX-1, and adhesion molecules; the degradation of IκBα; and the nuclear translocation of NFκB; in contrast, CSLE induced the expression of Nrf2 and HO-1 (P < 0.05 for all results).