Objective: To investigate the relationship between type 2 diabetes mellitus (T2DM) and the pathogenesis and metastasis of colorectal cancer. Methods: A case-control study was performed to compare 852 colorectal cancer patients with 940 controls (patients without cancer) recruited from 2001 to 2006, with respect to their sex, cancer subsite, the course of T2DM, hepatic metastasis, smoking and drinking. Correlated risk factors were analyzed. Results: The risk of colorectal cancer was increased in patients with T2DM and the relative risk (OR) was 2.466. The OR of male patients was higher than that of female patients, but with no significant difference (2.775 vs 2.070, P=0.394). The incidence of T2DM in patients with left hemicolon cancer was higher than that in those with right hemicolon cancer and rectal cancer, but with no significant difference between them. The colorectal cancer risk in T2DM patients with a DM course of 10 ～ 20 years was the highest, and the OR was 4.696. The rate of hepatic metastasis was higher in T2DM patients with colorectal cancer than that in celorectal cancer patients without T2DM and the OR was 2.888. Conclusion: T2DM may be one of the important pathogenic risk factors for colorectal cancer. The OR is increased with the extension of DM course within 20 years. Colorectal cancer patients with T2DM may be more prone to hepatic metastasis.