Allium cepa Linn (Onion) Organosulfuric compounds and phytonutrients have medicinal benefits. The study estimated the antioxidant effect of Allium cepa in fortified feed against oxidative damage caused by potassium bromate. Commercial feed was fortified by substituting 10 %, 20 %, and 30 % of rat's daily ration with the respective portion of pulverized Allium cepa. Potassium bromate was administered orally to the rats in all the groups except rats in the normal control. The rats in the test groups were allowed access to the fortified feed ad libitum. The animals were sacrificed; consequently, the serum, liver, and kidney were obtained for biochemical assay and histological assessment. The percentage composition of some amino acids and some proximate were higher in the fortified feed. Furthermore, Malondialdehyde (MDA) level in the liver, kidney, and serum decreased significantly (P ≤ 0.05) in rats fed with fortified feed compared to administered only Potassium bromate. Similarly, the concentration of total protein increased significantly (P ≤ 0.05) in the liver, kidney, and serum of the animals fed with fortified feed. The hematology result was normal in rats fed with fortified feed. The liver and kidney cell architecture was normal in animals fed with fortified feed. Allium cepa may have conferred protection and amelioration to oxidative damage by potassium bromate in rats.