DNA methylation, widely investigated as an epigenetic modification considered to be heritable and reversible, regulates gene expression extensively. It is well established that DNA methylation is a pivotal mechanism in regulating gene expression in the central nervous system. Recent research suggests that global methylation, methylation of specific genes, and abnormalities in pathways may contribute to the etiology of various mental disorders. In this chapter, we will discuss the mechanism and function of DNA methylation, genetic and environmental factors influencing methylation changes, and DNA methylation mechanisms observed in mental disorders, including obsessive-compulsive disorder, anxiety disorders, schizophrenia, mood disorders, depression, post-traumatic stress disorder, and neurodevelopmental disorders.