Chromosome 18p deletion syndrome or 18p- syndrome is a genetic condition caused by a deletion of genetic materials within chromosome 18, and the deletion involves the short arm of chromosome 18. It causes a wide range of medical and developmental concerns, such as mental retardation, short stature, holoprosencephaly, ptosis, small mandible, excessive caries, and dental anomalies. At present, the treatment for 18q- syndrome is only symptomatic. This article presents a case report: orthodontic treatment of a 14-year-old male patient with 18p- syndrome. He has a mental retardation, anomalous maxillary central incisor and small mandible. Due to macrodontia in maxilla and small mandible, there was a severe crowding. We proceeded with the extraction of upper and lower first premolars and a good alignment of teeth was obtained. But, because of mental retardation, the orthodontic treatment had a limitation. The special considerations of orthodontic treatment for the patient with 18p- syndrome are discussed