A 61-year-old man visited another hospital with chief complaints of dry cough, appetite loss and body weight loss. On chest X ray film and CT, right supraclavicular and mediastinal lymph nodes were swollen. For further examination, he was referred to our hospital and diagnosed as primary mediastinal choriocarcinoma. We conducted surgery after four cycles of chemotherapy using cisplatin, etoposide and bleomycin. After chemotherapy, residual tumors were found on chest CT. In FDG-PET some parts of the residual tumors seen on CT showed much less uptake and the others showed similar uptake. Histologically the areas showing similar uptake contained more viable cells, and the other areas showing less uptake almost completely consisted of necrotic tissue. We believe FDG-PET can be useful to evaluate the viability of tumors after chemotherapy in extra gonadal choriocarcinoma in the mediastinum.