BACKGROUND: Teratomas are the most common of germ cell tumor and have different epidemiology, histology, and biology than the others. These tumors occur equally in men and women are generally benign, occur in the mediastinum and compromise approximately 20% of anterior mediastinal masses. All forms of these tumors have a peak incidence in young adults. CASE REPORT: A huge mediastinal teratoma present in young girl 15 years old, a tumor located in the right chest with mass compression effect to spine, heart, and lung, produce prolong symptoms of dyspnea and chest pain. Delayed clinical presentation influences to post-operative complications such as prolong atelectasis, rupture of tumor mass which spoiled of tumor contents, empyema, and sepsis. CONCLUSION: Mediastinal mature teratoma with the delayed clinical presentation will give technical difficulties for resection and increase the risk of post-operative complications.