Abstract: This study is about a girl with chromosome deletion of 18q and with mental retardation and mild delay of physical development. Based on karyotyping of high resolution, florescence in situ hybridization (FISH) and microsatellite analysis mapping to 18q, we found that the patient’s karyotype was interpreted as 46,XX,del(18).(pter→q21:), ish del(18)(D18Z1+,qter-). Detection of D18S979 showed that the region from 18q21.1 to 18qter was deleted, which was originated from her father. There were MBP gene and GALNR gene in the deleted interval in which both of them were lost. In conclusion, deletion of 18q21→qter including the MBP gene and GALNR gene should be responsible for her mental re-tardation and mild delay of development.