Allium species such as onions and garlic are used as foodstuff, condiment, flavoring, and folk medicine. Onions may decrease hyperlipidemia and improve atherosclerosis. However, the ingredients in onion that are responsible for this phenomenon are not known. In the present study, we investigated the effects of cycloalliin, a sulfur-containing imino acid in onions, on lipid metabolism in Sprague-Dawley rats. When supplemented at the 0.1% and 0.3% levels to the atherogenic diet, cycloalliin reduced serum triacylglycerol (TAG) concentration by approximately 40% compared to the control. Serum cholesterol ester level also showed a tendency to decrease in cycloalliin groups. Hepatic lipid levels were comparable among the groups, although TAG and phospholipid contents were slightly higher in both cycloalliin groups. Dietary cycloalliin had no significant effect on hepatic enzyme activities responsible for TAG synthesis (phosphatidate phosphohydrolase, malic enzyme, and glucose-6-phosphate dehydrogenase (G6PDH)). In conclusion, dietary cycloalliin has serum TG-lowering effect without affecting hepatic TAG synthesis and content in rats, suggesting an alteration of lipoprotein assembly and secretion processes in the liver.