BACKGROUND Diabetic patients have a higher risk of colorectal cancer (CRC). The role of metformin in CRC incidence among type 2 diabetes mellitus (T2DM) remains controversial. OBJECTIVE A meta-analysis was performed to evaluate the role of metformin treatment in the occurrence of CRC among T2DM patients. METHODS Search was performed throughout PubMed, Embase, Springer databases up to November 2014. The search terms were (biguanides or metformin) and (bowel or colon or rectal or colorectal) and (cancer or neoplasm or neoplasia). Relative risk (RR) and 95% confidence interval (CI) was pooled using random-effects model or fixed-effect model basing on heterogeneity, which was calculated basing on Q statistics and χ2 test. In addition, subgroup analyses were performed according to region, study design and control treatment. Finally, publication bias was evaluated using Egger's regression test and trim and fill analysis. RESULTS A total of 11 studies, including eight cohort studies and three case-control studies, were enrolled in the meta-analysis. Obvious heterogeneity was noted, and a 25% lower CRC incidence was found among diabetic patients treated with metformin (pooled RR=0.75, 95% CI: 0.66-0.86), using the random-effects model. Subgroup analyses showed that CRC incidence significantly reduced among T2DM in different regions, non-metformin treatment and cohort studies. Evidence supported significant publication for studies investigating from Egger's regression test. Conversely, no missing data were found using trim and fill analysis. CONCLUSIONS In conclusion, the meta-analysis suggests metformin may reduce CRC incidence among diabetics, which is useful medical information for clinicians.