Synovial sarcoma is a malignant soft tissue tumor that primarily occurs in the praarticular regions, especially in the knee. They are composed of keratin-positive epithelial cells and vimentin-positive spindle cells. We report a 76 year old woman with a primary synovial sarcoma in the mediastinum that had severe adhesion to the right side of pericardium. Primary synovial in the midiastinum is extremely rare and this is the first case reported in the Korean literature. The mass including the pericardium was resected and the defect was closed with Gore-Tex patch. The patient is well and free of disease 6 months after the operation.