The aim of this study was to investigate effects of onion, red onion, or quercetin on plasma antioxidant vitamin, lipid peroxidation, and leukocyte DNA damage in rats fed a high fat-cholesterol diet. Forty SD male rats were assigned to normal control, high fat-cholesterol diet (HF), or HF+5% onion powder, HF+5% red onion powder, or HF+0.0l% quercetin. The HF diet resulted in significantly higher plasma lipid peroxidation which decreased with onion, red onion, or quercetin supplementation. Leukocyte DNA damage induced by HF diet decreased significantly in rats fed onion and red onion, while quercetin supplementation had no effect on preventing leukocyte DNA damage. $H_2O_2$ induced leukocyte DNA damage exhibited a highly significant negative correlation with plasma retinol and tocopherols. These results suggest that onion or red onion powder exerts a protective effect with regard to DNA damage in rats fed HF diet. However, 0.01% quercetin in pure form might not be effective at preventing DNA damage.