A Case of huge mucinous cystadenoma of borderline malignancy is presented. Due to recent development and application of technology in ultrasonography, we can find early ovarian cyst during or after pregnancy. This 28 year-old woman was admitted for an huge ovarian cyst with abdominal pain and mild dyspnea in 2 Jan. 2001. She has delivered a full term baby through vagina in Nov. 1999. and the huge ovarina cyst developed and growed after delivery very rapidly. An exploratory laparotomy disclosed huge ovarian mucinous cystadenoma of borderline malignancy. Salpingooophorectomy on the affected side and appendectomy were performed. She has been in good health during 4 month follow-up.