We report a 21 year-old girl with classical Rett syndrome (RS) based on clinical diagnosis. The molecular testing of MECP2 gene revealed that the patient is homozygous for a de novo 473C > T mutation, causing the T158M amino acid change. Chromosome analysis showed a normal karyotype, and the haplotype analysis ruled out the possibility of parental disomy or microdeletion in MECP2 gene. Cultured fibroblast analysis reveals a mosaic for the mutation. This is a documented case of a homozygous female with RS.