Collision tumor consists of two tumors occurring in the same organ without intermixture of the two cell types. The most common type of collision tumor in ovary is between teratoma and surface epithelial tumor. A 38-year-old woman presented with complained of lower abdominal pain and tightness, and a solid partially cystic left ovarian mass with minimal ascites was detected. Left salpingo-oophorectomy was performed. The ovarian mass measured 15 × 12 × 7 cm with a pedunculated mass on its surface which measured 6 × 2.5 × 2.5 cm. Histologically, it was a collision tumor of fibroma and mature cystic teratoma. Fibroma becomes more edematous as their size increases, which is frequently accompanied by the escape of increasing quantities of fluid from the tumor surfaces. Ascites is often detected when the fibroma is more than a diameter of 10 cm. It is important to identify the different components of a collision tumor for proper management.