A 47-year-old nulliparous, virginal woman presented to the emergency department with acute abdominal pain. Emergency pelvic ultrasound and abdominal CT were taken, which showed a significant amount of hemoperitoneum and a bicornuate uterus with about 18cm x 10cm mass on left uterus. Since the mass had increased vascularity and irregular margins, we thought that the mass could be a uterine sarcoma. MRI and PET/CT were taken additionally for oncologic evaluation before surgery. Intra operative findings showed a ruptured bicornuate uterus with a large mass within the left uterine horn. Total abdominal hysterectomy with bilateral salpingo-oophorectomy was performed. Pathologic analysis confirmed an undifferentiated uterine sarcoma. She was treated with 6 cycles of chemotherapy(etoposide, ifosfamide, cisplatin) postoperatively. Chest and abdomen CT for follow up after chemotherapy showed no sign of cancer recurrence. We suggest a bicornuate uterus with concomitant sarcoma should be concerned as a possible cause of uterine rupture by reviewing this case.