Malignant mesenchymoma is a very rare tumor presented during the embryonic and infant period and malignant mesenchymoma in the adult is extremely rare. Tumor is composed of two or more unrelated mesenchymal derivatives apart from fibrous tissue. These tumors are thought to be originated from embryonic mesenchyme capable of differentiating into any type of connective tissue. A 61 years old man with complaints of cough and copious sputum of onset of two months was admitted after initial examinations, showing a very huge mass over the right upper lobe. Right pneumonectomy with partial rib resection of 3rd, 4th, and 5th ribs was performed due to the initial diagnostic impression of squamous cell carcinoma by the fine needle aspiration biopsy. The operative field presented a mass locating across the interlobal fissure with severe adhesions to the chest wall. Postoperatively, the patient received 5,000 rads of radiotherapy and presently, 6 months later, has shown no signs of recurrence.