Diabetes mellitus (DM) is the most common metabolic disorder associated with high mortality, which is mostly due to its cardiovascular complications. Diabetic cardiomyopathy (CM) is characterized by abnormal ventricular function in the absence of DM-associated risk factors such as obesity, hypertension, hypercholesterolemia, or coronary artery disease. Oxidative stress plays a pivotal role in the development of diabetic CM, in which chronic hyperglycemia plays a major role. As this develops, the endogenous antioxidant system becomes suppressed and so cannot counter-balance the increased oxidative stress. The metabolic abnormalities of DM cause mitochondrial superoxide overproduction, which further enhances the production of other reactive species, including nitric oxide, hydroxyl radical, hydrogen peroxide and peroxy nitrite, causing aggravation of the myocardial damage. In addition, free-radical-mediated platelet activation in the narrowed arteries culminates in acute myocardial infarction and stroke, indirectly affecting cardiac function. This chapter focuses on various aspects of the oxidative stress induced by reactive species during the pathogenesis of diabetic CM.