Several evidences from longitudinal epidemiological studies have demonstrated that diabetes significantly increases the risk of Alzheimer's disease (AD). The underlying pathophysiologies that link diabetes and development of AD are, recurrent hypoglycemia, antioxidative stress due to hyperglycemia, atherosclerosis and vascular lesions, and abnormal insulin signaling in the brain. Emerging evidences from neuroimaging studies of diabetic patients can shed light on the pathomechanisms linking diabetes with AD. In this review, recent neuroimaging findings for diabetes are described. Diabetic patients have diffuse cortical atrophy with notable vulnerability in the temporal lobe. Hyperinsulinemia and hypoglycemia increases cerebral blood flow and metabolism, while hyperglycemia decreases. Diabetes may facilitate the onset of AD via vascular lesion, poor vascular responsiveness, and aberrant amyloid metabolisms, probably through atherosclerosis. Studies on the brain of patients with diabetes may provide clues for preventive therapy in AD.