Type 2 diabetes mellitus (T2DM) is characterized by chronic hyperglycemia, which induces neuron, kidney and eye dysfunction. Hyperglycemia is also a risk factor for dementia. T2DM also causes brain insulin resistance, oxidative stress and cognitive impairment. Futhermore, oxidative stress causes a complex dysregulation of cell metabolism resulting in insulin resistance and beta cell dysfunction. Alzheimer’s Disease (AD) is the most common form of dementia among older people worldwide, and oxidative stress is an important pathogenic factor in AD. Oxygen metabolism generates free radicals such as hydroxylradical, superoxide radical, and reactive nitrogen species, inducing ROS. An imbalance between oxidant and antioxidant agents could generate oxidative stress, which damages macromolecules and disrupts the reduction/oxidation (redox) signaling. Mitochondria contain many redox enzymes, and generate ROS when there are inefficiencies in oxidative phosphorylation. Mitochondrial dysfunction occurs early and has a primary role in the pathogenesis of AD. T2DM patients show increased incidence of AD, and one report has shown the role of insulin in that it links T2DM with AD through mitochondrial alterations, oxidative stress and glucose metabolism. The insulin effect not only impacts glucose metabolism, but also has neuroprotective and neuromodulatory effects.