An 18 years old female patient suffered from cerebrovascular occlusive disease, moyamoya disease, showed a huge cyst in her left mandibular body in the radiological observation. The lesion was asymptomatic and found during routine dental check. She had no experience of traumatic injury on her jaw. The cystic lesion was ovoid with irregular scalloping margin and multilobular image, and occupied the whole marrow space of mandibular body with slight expansion of buccal cortical bone. During operation the lesion showed an empty space covered with grayish white fibrous tissue. The luminal fibrous tissue and underlying bony tissue were curettaged and examined pathologically. In the histological observation the lesion was a pseudocyst lined by thick fibrous tissue. Some large vessels underwent atherosclerotic change, exhibiting thickened vessel walls which were partly distorted with hemorrhage and thrombi, and some small capillaries were extremely dilated with hemorrhage and subsequently resulted in perivascular ischemic change with chronic vasculitis. This mandibular cystic lesion was finally diagnosed as simple bone cyst (SBC) associated with moyamoya disease differentially from aneurysmal bone cyst (ABC), traumatic bone cyst (TBC), periapical odontogenic keratocyst, and central giant cell granuloma. Therefore, it was presumed that the thromboembolic and atherosclerotic vessels of moyamoya disease might increase the hemodynamic pressure of mandibular bone marrow tissue and subsequently was able to induce SBC.