Garlic is regarded in many areas as a folk medication. In recent years, dietary supplements containing garlic have become very popular because they are advertised as offering protection against cancer, heart and other diseases. Garlic is also known to possess antioxidant activity. In this work, oleic acid was oxidized by a free radical initiator and HPLC-chemiluminescence detection was used to detect hydroperoxide formation. Various preparation of garlic extracts were investigated in this system. Our data showed that a methylene chloride extract of freshly homogenized garlic has a strong antioxidation activity. However the antioxidant activity is nearly completely lost when fresh garlic was boiled in water or treated with strong acid before homogenization. These data suggest that allicin but not alliin possess antioxidation activity. Diallyl disulfide has significantly less antioxidant activity than allicin.