This study was performed to evaluate the antioxidant activity of silymarin against human low density lipoproteins(LDL) oxidation. Silymarin extracted from Silybum marianum was successively purified with solvent fractionation and followed by silica gel column chromatography. The active substances were separated by HPLC and the isolated active substances, silymarin were identified by IR, NMR, GC-MS as silymarin. Silymarin inhibited at the 5 $mu$M Cu2+-mediated oxidation of human low density lipoprotein (LDL) in a dose dependent manner. Silymarin completely inhibited LDL oxidation at 50 $mu extrm{g}$/$mell$ concentration. These findings suggest that silymarin may protect LDL against oxidation in atherosclerotic lessions.