Introduction A teratoma is a tumor with tissue or organ components resembling normal derivatives of more than one germ layer. Although the teratoma may be monodermal or polydermal (originating from one or more germ layers), its cells may differentiate in ways suggesting other germ layers. The tissues of a teratoma, may be quite different from surrounding tissues and may be highly disparate; teratomas have been reported to contain hair, teeth, bone and, very rarely, more complex organs or processes such as brain matter, eyes, torso, and hands, feet, or other limbs. Because they are encapsulated, teratomas are usually benign, although several forms of malignant teratoma are known. A mature teratoma is typically benign and found more commonly in women, while an immature teratoma is typically malignant and is more often found in men. Teratomas are thought to be present at birth, but small ones often remain undiscovered until much later in life. Case A previously healthy 11-year-old female child was admitted complaining of abdominal distension for 2 months. On physical examination, abdomen was rigid and distended with usual bowel sound. Laboratory findings showed Hb 12.7 g/dL; hematocrit(Hct) 37.9%; WBC count 8,310 μL; platelets count 321,000 μL; AST(GOT) 16 IU/L; ALT(GPT) 8 IU/L; CRP 1.30 mg/L. Abdomen-pelvic with-contrast CT indicated 30x25cm-sized multi-lobulated cystic mass with septum and multiple, solid, calcification with fat component causing compression of IVC and displacement of uterus for anterior aspect, which implied immature cystic teratoma. On the 2nd hospital day, the patient was transferred to gynecologic department and had an open surgery of right adnexectomy. Mature cystic teratoma of right ovary was diagnosed at the biopsy. 3 days after surgery, all symptoms of the patient were relieved and the patient was discharged. At the follow up of one week after discharge, abdominal distension was not checked and no other symptoms were checked. Conclusion A 11-year-old child complained of abdominal distension which persisted for 2 months. A huge, multi-lobulated cystic mass was detected at the abdominal CT which indicated teratoma, clinically. Mature cystic teratoma was diagnosed at the biopsy after right adnexectomy. The author reports a case of mature cystic teratoma in a child with abdominal distension.