Atypical teratoid rhabdoid tumor is a rare highly malignant embryonal tumor of the central nervous system that is often seen in early childhood. It is very important to distinguish it from other brain tumors because it has a very poor prognosis and there are differences in its treatment. A case of atypical teratoid rhabdoid tumor in a six-week-old male baby is presented. The tumor was located at posterior fossa. Histopathologically, the tumor has rhabdoid tumor cells and mesenchymal components beside the undifferentiated small cells. While EMA, vimentin, synaptophysin and smooth muscle actin have been stained with immunohistochemical staining, desmin, chromogranin, CD 99 and CD 56 have not been stained. The patient died four months after surgery despite the chemotherapy given. In conclusion, morphological characteristics can vary to a large extent that it is difficult to recognize atypical teratoid rhabdoid tumor. Immunohistochemical panel and molecular genetic study will help to establish the correct diagnosis.