Primary non-Hodgkin's lymphoma (NHL) arising from the posterior mediastinum is rarely reported in the literature. Here we presented a 75-year-old male who called medical attention because of drug-induced hypoglycemia. A chest roentgenogram incidentally disclosed a mediastinal mass. The computer tomography scanning of chest revealed a posterior mediastinal mass with pleural effusioin. The thoracoscopic biopsy was done. The histology and immunohistochemical studies of specimen identified a malignant lymphoma, diffuse large cell, B cell in nature. No other lesion was found after the subsequent work-up. He was treated with chemotherapy of CHOP. Complete remission was achieved after three courses of CHOP. Then he completed total six courses of chemotherapy. He continued to be disease-free survival and followed up at our OPD.