Background: Over the years, there has been research on the anti-obesity effect of the Allium cepa bulb, but a dearth of research was carried out on the leaves, which is consumed as vegetable salad and soup, hence this study was proposed.Objective: This study aims at investigating the effect of A. cepa leaves on high fat diet (HFD) induced obesity in male Wistar rats. Methods: Twenty-eight male Wistar rats were divided into four groups with seven rats each. Apart from Group 1(normal control which received normal pelleted diet), obesity was induced in 21 rats of Group 2 to 4 with HFD. Group 2, the obese control was administered with 100% HFD, while the diet for group 3 and 4 was supplemented with 10% and 20% A. cepa powdered leaves, respectively, for 28 days. Results: In the rats treated with 10% and 20% A. cepa, body weight, fat mass, blood glucose, total cholesterol,triglycerides, aspartate amino transferase, alanine amino transferase, creatinine and urea levels were reduced significantly (P < 0.05) in comparison with the obese control group. The liver of the rats treated with 10% and 20% A. cepa leaves revealed small and few amounts of fat deposits in comparison with the obese control group, which revealed numerous and large deposits of fat. The kidney of the rats treated with 10% and 20% A. cepa leaves showed moderate and mild inflammation, respectively, in comparison with the obese control group which showed acute inflammation. The leaves of A. cepa had antioxidant properties and the presence of volatile compounds with anti-obesity properties in A. cepa were identified using gas chromatography-mass spectrometry(GC-MS). Conclusion: A. cepa leaves had weight-loss effect in that it decreased body weight, fat mass, glucose and lipid levels including fat deposits in the liver.