ABSTRACT: Three antioxidant flavone C-glucosides, isoorientin, vitexin, and isovitexin, were identified for the 1st time from Atractylodes japonica leaves by an activity-guided fractionation with various analytical techniques including column chromatography, high-performance liquid chromatography (HPLC), fast atom bombardment mass spectrometry (FAB-MS), and nuclear magnetic resonance (NMR). Hydroxyl and superoxide anion radicals scavenging activities of these compounds were examined using electron spin resonance (ESR). Inhibitory activities of these compounds on human low-density lipoprotein (LDL) oxidation were evaluated by malondialdehyde (MDA) and thiobarbituric acid-reactive substances (TBARS), both representing intermediates of lipid peroxidation. These flavone C-glucosides displayed about 50% of scavenging activity against hydroxyl radicals at the concentrations below 10 μM. The superoxide dismutase (SOD)-equivalent activities of isoorientin, vitexin, and isovitexin at 1 mM were 31.37, 2.71, and 2.63 unit/mL, respectively. Isoorientin at 1 μM exhibited over 60% inhibition of MDA formed during copper-mediated human LDL oxidation. Amounts of free MDA in LDL treated with isoorientin, vitexin, isovitexin, and control were 20.06, 40.73, 34.08, and 48.03 nM/mg protein, respectively. These compounds also prolonged the lag phase time of the conjugated diene formation. There was a positive correlation between the free radical scavenging activities and the inhibitory effects on the LDL oxidation of these compounds. These results suggest that the flavone C-glucosides isolated from the leaves of A japonica possess beneficial antioxidant properties against free radicals as well as LDL oxidation.