Colorectal cancer is extremely rare in children. Unlike adult colorectal cancer, the overall prognosis of colorectal cancer in children is poor. Delayed diagnosis, advanced stages of the disease at presentation, and mucinous type of histology are the major determinants of poor outcome in childhood. A 13-year-old boy with abdominal pain visited our hospital. Physical examination andabdominal ultrasonography identified acute appendicitis with perforation. He underwent appendectomy and then the pathologic findings revealed mucinous adenocarcinoma. The cancer was located at the transverse colon and had metastases on peritoneal wall at 2nd laparotomy. Extended right hemicolectomy was performed. He underwent palliative chemotherapy. After 4 months later, hepatic metastasis and aggravated peritoneal seedings developed. He died of renal failure and pneumonia 13 months after operation. We need to have a high index of suspicion for the possibility of a malignant colorectal tumor in any childhood case with nonspecific signs and symptoms.