The leiomyosarcoma is uncommon tumor and commonly arise from retroperitoneum or mesentery. But leiomyosarcoma arised from mediastinum is very rare and only incidental case report is present. Mediastinal leiomyosarcoma may originate from superior vena cava, pulmonary artery, small vessels of alveoli, esophagus and cardiac muscle. Common symptoms related with leiomyosarcoma of mediastinum are cough and dyspnea. And dysphagia, chest pain, hemoptysis and pleural effusion can be rarely produced. Although complete resection of tumor was the choice of treatment, there was no effective therapy in patients who had disseminated disease or huge tumor mass. We report a case of posterior mediastinal leiomyosarcoma confirmed by immunohistochemical staining and electron microscopic finding with a review of literature.