The antioxidant activity of the curcuminoids of Curcuma domestica L. and C. xanthorrhiza Roxb. and eight compounds which are prevalent constituents of their rhizome oils were investigated in an effort to correlate human low-density lipoprotein (LDL) antioxidant activity with the effect of the herbs and their components. The antioxidant activity was examined using thiobarbituric acid reactive substances (TBARSs) assay with human LDL as the oxidation substrate. The methanol extracts and rhizome oils of C. xanthorrhiza and C. domestica showed strong inhibitory activity on copper-mediated oxidation of LDL. Curcumin, demethoxycurcumin, and bisdemethoxycurcumin, isolated from the methanol extracts of both plants, exhibited stronger activity than probucol (IC(50) value 0.57 μmol/L) as reference, with IC(50) values ranging from 0.15 to 0.33 μmol/L. Xanthorrhizol, the most abundant component (31.9%) of the oil of C. xanthorrhiza, showed relatively strong activity with an IC(50) value of 1.93 μmol/L. The major components of C. domestica, ar-turmerone (45.8%) and zerumbone (3.5%), exhibited IC(50) values of 10.18 and 24.90 μmol/L, respectively. The high levels of curcuminoids in the methanol extracts and xanthorrhizol, ar-turmerone and zerumbone in the oils, and in combination with the minor components were responsible for the high LDL antioxidant activity of the herbs.