Soft tissue giant cell tumor (GCT) is rare. It usually involves the extremities. We report the case of a 37-year-old woman who was suspected of having mediastinal tumor on radiograph. Thoracic CT revealed the tumor had extensive calcification and invaded the adjacent vertebrae and spinal canal. It intensively accumulated Tc-methylene diphosphonate on bone scan. The tumor showed hypointensity on T1-weighted and mixed intensity on T2-weighted fat-saturated sagittal images. Finally, a soft tissue GCT was confirmed by pathology. The case cautions us soft tissue GCT should be in the differential diagnosis spectrum in a calcified posterior mediastinal mass with Tc-methylene diphosphonate accumulation.