Castleman's disease is a slowly progressive and rare lymphoproliferative disorder. Here, we report a 55-year-old woman with superior mediastinal Castleman's disease being misdiagnosed for a long term. We found a 4.3 cm mass localized in the superior mediastinum accompanied with severe clinical symptoms. The patient underwent an exploratory laparotomy, but the mass failed to be totally excised. Pathologic examination revealed a mediastinal mass of Castleman's disease. After radiotherapy of 30 Gy by 15 fractions, the patient no longer presented previous symptoms. At 3 months after radiotherapy of 60 Gy by 30 fractions, Computed tomography of the chest showed significantly smaller mass, indicating partial remission. Upon a 10-month follow-up, the patient was alive and free of symptoms.