A rhabdomysarcoma originating during childhood is an easily-detected soft tissue sarcoma which frequently occurs in the head, and neck, or genitourinary system. It has been reported that in the mediastinum its rate of occurrence is 1 percent, though the rate at which it occurs in the posterior mediastinumhas not been reported. We recently encountered a case of embryonal rhabdomyosarcoma of the posterior mediastinum in a 12-year-old girl with neurofibromatosis type 1. Initial chest X-rays revealed a well-marginated round mass, with destruction of the right posterior rib. CT scans indicated the presence of an iso-dense mass which after contrast infusion showed heterogeneous enhancement. T1-and T2-weighted MR images showed, respectively, slight and heterogenous hyperintensity, with invasion of the thoracic spine and compression of the spinal cord. After surgery, embryonal rhabdomyosarcoma of the posterior mediastinum was pathologically confirmed.