It is often difficult to classify rare malignant pediatric mixed brain tumors composed of mesenchymal elements. A 6-year-old boy presented to our hospital with a left frontal massive tumor manifesting as complaints of rapidly progressive right hemiparesis and motor aphasia over 2 weeks. Computed tomography showed a left frontal mass with thick calcification. Magnetic resonance imaging revealed an enhanced lesion with perifocal edema and mass effect. Total removal of the tumor was performed. Histological examination of the resected tumor revealed an anaplastic malignant small round cell component with a bland-looking mesenchymal spindle cell component. The patient was treated with irradiation to the whole craniospinal axis and a boost to the tumor bed, followed by chemotherapy consisting of ifosfamide, cisplatin, and etoposide, resulting in good control without local recurrence or metastasis at 2 years. A combined malignant tumor composed of ectodermal and mesenchymal components is generally named malignant ectomesenchymoma (MEM). The more malignant part of MEM is the mesenchymal component in most cases. In the present case, the more malignant component was not the mesenchymal component, but the small round cells.