Diabetes mellitus is a disease having a characteristic symptom of high level of sugars in the blood (hyperglycemia), perceptible through the urine due to failure of metabolism, and thus, the name. Controversy exists about whether increased oxidative stress is merely associative rather than casual in diabetes mellitus. Several mechanisms may cause oxidative insult in diabetes mellitus, although their exact contributions are not entirely clear. Accumulating evidences point towards many interrelated mechanisms that increase the production of reactive oxygen and nitrogen species or decrease antioxidant protection in diabetic patients. Therefore, their role in diabetes mellitus needs critical examination. This review aims to summarize the role of oxidative stress in diabetes mellitus.