Arachnoid cyst is a relatively common congenital intracranial lesion and often accompanies with chronic subdural hematoma. In case of coexisting with chronic subdural hematoma, arachnoid cyst is easily confused to hematoma, which may need surgical exploration. We report a case of chronic subdural hematoma accompanied with arachnoid cyst which was treated by trephination only. In addition, we report the radiologic features of this patient.