Malignant mesenchymoma is a soft-tissue tumor arising preferentially in the extremities and the retroperitoneum. We report a case of primary malignant mesenchymoma of the left side of the posterior mediastinum with local recurrence on both sides of the mediastinum. A 24-year-old woman was admitted to our hospital complaining of cough. Chest X-ray showed a giant mass in two-thirds of the left lung field, and computed tomography of the chest revealed a lobulated large mass with a fat-density area and a calcified spot in the mediastinum and left pleural space. Magnetic resonance of the chest demonstrated a large, solid mass consisting mainly of areas with the same intensity as fatty tissue and partly of areas of heterogeneous moderate intensity. The tumor was resected completely through a left posterolateral thoracotomy. The pathologic diagnosis was malignant mesenchymoma: well-differentiated liposarcoma with osteocartilagenous differentiation. The patient had local recurrence in both sides of the mediastinum 3 years and 6 months after surgery, and the recurrent tumors were completely resected again.