Germ cell tumors (GCTs) are a family of neoplasms with diverse histopathological, clinical features and prognosis, which are widely considered to arise from almighty primordial germ cells⁽¹⁾. Gonads are the most common organ that GCTs predominantly occurs, while a small proportion of GCTs primarily develops in extra-gonad regions such as pineal gland, coccyx, mediastinum, retroperitoneum⁽²⁾ and the anterior mediastinum is the prior anatomical position in which approximately 10–20% of primary extra-gonad GCTs occur⁽³⁾. Pathologically, GCTs are divided into two categories: pure seminomas and non-seminomatous germ cell tumors (NSGCT)⁽⁴⁾. Teratoma, a subtype of NSGCT, accounts for about 40% - 60% of all mediastinal GCTs with the majority of its histology being mature⁽⁵⁾. While the immature teratoma, a particularly rare subtype of teratomas with poor clinical prognosis, accounts for only approximately 1.8% of mediastinal GCTs and about 4% of mediastinal teratomas⁽³⁾. Moreover, the immature teratoma with somatic-type malignant transformation (SM) is extremely rare⁽⁶⁾, and there are only a few cases described about this topic^(7-12). We herein report a case of a chemotherapy-resistant and extremely giant immature teratoma who received en bloc resection with pathologic findings of foci of sarcoma and poorly differentiated squamous cell carcinoma. To our best knowledge, this is the largest primary mediastinum immature teratoma with somatic-type malignant transformation treated with an en bloc resection so far.