Synovial sarcoma is the third most common histological type of extremity soft tissue sarcoma. However, primary mediastinal synovial sarcoma is extremely rare. We present 2 cases of unresectable primary mediastinal synovial sarcoma. The radiographic imaging of our present cases was characteristic of a heterogeneously enhancing mass. They were treated with radiotherapy and chemotherapy. However, there was complete obstruction of esophagus resulting from progressive diseases. The radiographic findings and treatment were discussed.