Objective To assess the effects and safety of sitagliptin combined with metformin in treating type 2 diabetes mellitus. Methods The Cochrane Library, PubMed, EMbase, CNKI, WanFang Data, VIP and CBM were searched to collect the randomized controlled trials (RCTs) on sitagliptin combined with metformin in treating Type 2 diabetes mellitus (T2DM) from inception to November, 2012. References of included studies were also retrieved. Two reviewers independently screened studies according to exclusion and inclusion criteria, extracted data, and assessed the methodological quality. Then, meta-analysis was performed using RevMan 5.1 software. Results 7 RCTs involving 2 917 patients were included. The results of meta-analysis showed that, compared with metformin alone, sitagliptin combined with metformin effectively improved HbA1c levels (WMD= –0.62%, 95%CI –0.76 to –0.47, Plt;0.000 1) and fasting plasma glucose levels (WMD= –0.7 mmol/L, 95%CI –1.03 to –0.37, Plt;0.000 01), and increased insulin sensitivity and β-cell function. But there was no significant difference between the two groups in the incidences of gastrointestinal reactions and hypoglycemia. Conclusion Compared with using metformin alone, sitagliptin combined with metformin can improve glycemic control, enhance insulin sensitivity and better β-cell function more effectively and both have a similar effect on weight lose, but there is no significant difference he incidences of gastrointestinal reactions and hypoglycemia. The above conclusion should be verified by more large-scale high-quality studies in future due to the limitations of the methodological quality and sample size of the included studies.