Malignant melanoma develops from the melanocyte and the most common primary site is skin, followed by mucosa and retina. Even though any other tissue where melanocytes reside could be the primary site of the malignant melanoma, the one developed in the mediastinum is rarely reported. We experienced a patients of 54 years old woman whose initial symptom was progressive dyspnea for one month, and proved to have the anterior mediastinal mass with pleural effusion and the small mass in the abdominal soft tissue. The needle aspiration biopsy from the mediastinal mass showed the consistent findings with malignant melanoma. We concluded the mediastinum was the primary site of the malignant melanoma of this patient because we couldn't find any other evidence of primary tumor in skin, oral and gastrointestinal mucosa, and retina. She has been treated with combined chemotherapy with dacarbazine, cisplatin and vinblastine. Her symptom was improved after chemotherapy and follow up chest CT after three cycles of chemotherapy showed the decreased tumor size in the mediastinum.