This paper reports a rare case of rectosigmoid colon carcinoma metastasizing to anal fistula. The patient is a 57-year old man with a 7-year history of a fistula in ano. Colonoscopy revealed a rectosigmoid carcinoma. He underwent high-anterior resection. Two years and 3 months later, a subcutaneous tumor was found at the external opening of fistula. Trans perineal tumor excision plus fistulectomy was performed. Pathology revealed that the colon cancer and the perianal tumor were both moderately differentiated adenocarcinoma. Many reports support the concept of tumor cell implantation in mucosa that have been altered or denuded by various factors. Sixteen reports could be found of implantation metastasis of colorectal cancer into anal fistula. In diagnosis, it is important to differentiate implantation of colorectal cancer in anal fistula from primary anal fistular adenocarcinoma. The histology of the perianal tumor in this patient closely resembles the tumor of the colon. Furthermore, immunohistochemistry for cytokeratins 7 and 20 was performed on tissues to distinguish colorectal adenocarcinoma from anal gland carcinoma. Both colorectal cancer and perianal tumor showed CK7-/CK20+.