Primary adenocarcinoma of the vermiform appendix is rare clinical entity, fewer than 200 cases having been reported. The diagnosis is infrequently made preoperatively and is usually made by the pathologist. Primary adenocarcinoma of the appendix is a three varieties; carcinoid, mucinous cystaden- ocarcinoma (malignant mucocele), and colonic type adenocarcinoma. This patient was 58 year-old housewife who had suffered from right lower quadrant abdominal pain for 3 days. On physical examination, she showed localized right lower quadrant tenderness and small egg sized firm mass was noticed on right lower abdomen. During operation, there was a enlarged very thick fibrotic wall which was filled with mucinous material at the tip of appendiceal site and measured 5x4x4cm. Appendectomy was done. The dilated appendix showes proliferation of atypical glands lined by multiple layer of columnar cells, scattered nests of mucin pools lined by atypical cells in muscular wall on microscopic findings. So, this lesion of appendix was diagnosed to mucinous cystadenocarcinoma. After 7 days from appendectomy, right hemicolectomy was done. Her condition is a quite well, and she is enjoying in her life now.