This is to report a case of traumatic bone cyst which occurred in the left condylar head of the mandible. It is a very rare case and the prognosis after operation seems to be fair, therefore we report as follows. 1. The pt, a 49 year-old female, has been suffering from difficulties of mastication with disfiguration of face due to deviation of mandible to the right side for 20 years. 2. Histopathologic findings showed the figure of multilocular bony cysts contained with fibrous tissue, osteoid tissue and many hematopoietic cells 3. Roentgenographic features showed well circumscribed radiolucent lesion with some radiopaque area. 4. After clinical evaluation, condylectomy was done to remove the lesion. 5. After the operation, the esthetic problem and masticatory functions were improved.