PURPOSE: Malignant peripheral nerve sheath tumors most often arise from the anatomically discernible peripheral nerve or neurofibroma. METHODS: A 55-year-old man had a rapidly growing pedunculated large mass on the sacrolumbar junction for 2 years. He has congenital neurofibromatosis type I. He had multiple caf-au-lait spots and multiple neurofibromas on the entire body. The mass developed from a subcutaneous nodule on the sacrolumbar junction and grew rapidly. The preoperative punch biopsy revealed a malignant peripheral nerve tumor. The mass was completely excised with 1 cm free margin above the deep fascial plane. RESULTS: There was no evidence of recurrence of tumor for 19 months of follow-up examination. CONCLUSION: Malignant peripheral nerve sheath tumor is very rare and has unique feature. We report a successful case of malignant peripheral nerve sheath tumor with the review of the literatures.