Commensal microflora engages in a symbiotic relationship with their host, and plays an important role in the development of colorectal cancer (CRC). Pathogenic bacteria promote chronic intestinal inflammation and accelerate tumorigenesis. In sporadic CRC, loss of an effective epithelial barrier occurs at early stage of CRC development. As a result, non-pathogenic bacteria and/or their products infiltrate tumor stroma, drive "tumor-elicited inflammation" and promote CRC progression by activating tumor-associated myeloid and immune cells that produce IL-23 and IL-17. In this article we will summarize the recent advances in understanding the relationship between gut flora and CRC.