Malignant epithelial ovarian tumors are common in adult women, but are rare in childhood. Especially, mucinous ovarian cystadenocarcinoma are extremely rare in pediatric age group, only a limited number of cases are reported in this age group till date with no case reported in preadolescence age in the literature. We describe a case of mucinous cystadenocarcinoma in eleven year old premenarchal girl presenting with symptoms of abdominal distension and respiratory distress from last three months.