Obesity is a noticeable public health problem worldwide. It is well known that obesity affects physical health and cognitive function. It impairs all domains of cognitive function. Moreover, obesity affects both metabolic activity and cerebral structure of the brain. Bariatric surgery is surgical procedures that help patients with extreme obesity to lose weight. Multiple domains of cognitive function improve 24 to 36 months after bariatric surgery. Mechanism of such improvement is still questionable. It may be related to improvement of underlying medical condition, improved glucose regulation and insulin sensitivity, or correction of leptin and ghrelin systems. In this article, we review the effect of bariatric surgery on cognitive function and its possible mechanism.