Rett syndrome is an X-linked dominant, progressive neurodevelopmental disorder, with a prevalence estimated to be one in 10,000-15,000 girls, which is thought to be the second most common genetic causes of mental retardation in females after Down syndrome. Patients with classic Rett syndrome show an apparently normal neonatal period, followed by developmental regression and deceleration of head growth, accompanied by gradual loss of speech and purposeful hand use, and development of microcephaly, seizures, autism, ataxia, intermittent hyperventilation and stereotypic hand movements. After regression between infancy and the fifth year of life, the clinical course stabilizes and patients usually survive into adulthood. It was recently discovered that Rett syndrome is caused by mutations in the methyl-CpG binding protein 2(MECP2) gene. Diagnosis of Rett syndrome is clinically difficult before three years of age, especially in atypical cases, but molecular analysis of the MECP2 gene could assist correct diagnosis in some patients. Recently, we diagnosed a case of Rett syndrome in a two year-old girl by mutational analysis of the MECP2 gene and want to report this case with brief review of literature.