Many systemic reviews and meta-analysis indicate that preexisting diabetes increased all types and sitespecific (e.g., colorectal, breast, gastric, pancreatic, renal and prostate) cancer incidences and mortalities. In addition, diabetes treatments were also related with either increased or reduced risks of cancer. Metformin showed a potential cancer protective effect significantly decreased hepatocellular carcinoma, colorectal,pancreatic, gastric and esophagus cancer for diabetic patients. However, exogenous insulin and sulfonylureas were associated with the risk of breast, colorectal, pancreatic cancers and hepatocellucar carcinoma.Thiazolidinediones were related with the decrease of hepatocellucar carcinoma. In this article, we review the association between cancer risk and diabetes and the evidence on the therapeutic effect of metformin.