Aim: To investigate the role of oxidative stress in the pathogenesis of Alzheimer disease, and study the present status of melatonin in preventing and treating Alzheimer disease. Methods: The studies about the evidence and possible mechanism about the oxidative damage involved in Alzheimer disease, and the studies about the present status of the application of melatonin in the treatment of Alzheimer disease were analyzed. Results: Senile plaques, neurofibrillary tangles and neuronal death, in which free radicals are all involved, represent the three pathological features of Alzheimer disease. Therefore, the application of ectogenous antioxidant is a potential method for the drug treatment of Alzheimer disease. Melatonin, which is the main hormone secreted by pineal gland, plays a role against free radicals and in protecting neurons, and its role in delaying aging has been definite, and it has also been proved that melatonin is decreased in patients with Alzheimer disease. Conclusion: Oxidative damage plays an important role in the attack of Alzheimer disease. Melatonin is powerful against free radicals, but whether the ectogenous supplement of melatonin can prevent or relieve the attack and progress of Alzheimer disease needs further study.