The authors report a case of arachnoid cyst arisen in the middle cranial fossa combined with intracystic hemorrhage. The pre-operative diagnosis was confused with chronic subdural hematoma. The post-operative final pathologic diagnosis was arachnoid cyst. The plain skull and C-T findings are illustrated. The light microscopic features of arachnoid cyst are presented. The brief description of the treatment and its prognosis are given.